You Have No Recourse
You have no recourse, at least to the extent that the people communicating to your employer point out the particular Facebook posts that were made or accurately summarize or paraphrase them (if the content of the Facebook posts was misrepresented to the employer that would be a different story).
A factually truthful statement (e.g., "McGovern made a Facebook post at this link which you ought to look at") is absolutely privileged from legal liability under the 1st Amendment to the U.S. Constitution (assuming that you are talking about a U.S. case).
The only exceptions would be when (1) the information provided is false and the person providing the tip has the appropriately standard of intent1 in providing the false statement (which can be actionable as defamation or intentional interference with contract as the DJ suing Taylor Swifts alleges in the Mueller v. Taylor Swift trial pending right now), or (2) the person providing the tip was someone who owed you a duty of confidentiality (e.g. your lawyer or psychiatrist), but that duty applies only to confidential communications between you and that person in most cases, and hence not to Facebook posts.
Also, whether your own statements on Facebook were true, false or neither (e.g. statements of opinion) is irrelevant to the culpability or liability of the people providing the tip to your employer, so long as their description of your posts made to the employer were substantially accurate or to the extent that they were inaccurate were accompanied by a means by which the employer could determine what you actually said without relying on their second hand account.
1 To the extent that you are not a public figure and this is not a matter of public concern, even a negligent misrepresentation about what you actually wrote on Facebook, that is not a statement of opinion and is not accompanied by a reference allowing someone to confirm the accuracy of the statement at the source, could give rise to legal liability on the part of the person giving the tip. The standard of intent is the higher "actual malice" threshold for statements on matters of public concern, statements by media defendants and statements about public figures. But, there is nothing in the question to suggest that the tip given was in any way inaccurate.
This Does Not Legally Constitute Harassment
This is not harassment in a legal sense, even if it may have been an unkind thing to do on the part of the person doing it done with a specific and malicious desire on the person providing the tip to harm you. This is a natural consequence of you saying something on the Internet, which is forever. If your employer doesn't like true things that you say whether or not they relate to your job, if you are an employee at will you can be fired for it.
It isn't absolutely impossible for factually true statements to constitute harassment, in a legal sense, but in those cases, it is the frequency and character of the communications, rather than their content or intended recipient, that make them harassing.
For example, if the person providing the tip sent an email about that tip every five minutes for several days to every email address at your employer, causing your employer to find that it was just too disruptive to the employer's business to keep you on the payroll, that might be harassment, but that would have nothing to do with the content of the message provided in that case.
Similarly, if someone screamed and yelled their statement over a loudspeaker every time you tried to tutor someone, that might be harassment, again, without regard to the content of what they were saying. In those cases, the "time, place and manner" exception to laws limiting the freedom of speech would apply.
Could An Employer Take Employment Action Based On These Posts?
There are a couple of states (Wyoming and Colorado, at least) which prohibit employers for taking employment actions against employees for lawful off the job conduct, despite the fact that otherwise, the default rule of law is that an employer can normally treat an employee at will (which the vast majority of employees are) as the employer deems fit. But, even then, liability is limited to the employer and not the person providing the tip. And, even in those states, off duty conduct can sometimes be considered by employers when it reflects directly on your fitness and ability to do your job when you are on the job, which a post about mathematics made by a math tutor very well might.
Some high level employees with written contracts, career government employees with civil service protections, and almost all unionized employees can only be fired or disciplined on the job when the higher standard of "for cause" employment action is met, which posting something true on Facebook would not normally be. But, in the "at will" employment world, anything you say can and will be held against you even if it is entirely true. But, again, even if the employer misuses the information that the employer receives, that doesn't mean that someone who provided truthful information to your employer has any legal responsibility whatsoever to you or that the person providing the tip has done anything legally wrong in any way.
For the most part, even if every word of your Facebook post was 100% true and accurate, outside the couple of states mentioned, you can be fired for posting it anyway if you are an employee at will.
For example, suppose that your employer thinks that the Kumon method of teaching mathematics is a horrible method of instruction, and you post Kumon method exercises or explanations on your Facebook page (assume to avoid going on tangents that you had express permission from the copyright holder to do so). You could still be fired by your employer for posting that on Facebook in the vast majority of states.
A Footnote on Internet Age Defamation Jurisdiction
The usual rule is that a U.S. Defendant can be sued for defamation or similar torts where a statement is intentionally published to by the person being sued. In the case of a media defendant, or someone suing over a post made on Facebook, that could be any place in the world in many cases.
But, while the geographic location of your Facebook post may be unclear, the geographic location of your employer to whom the statement you are concerned about being communicated was made, is not. Any harm done to you was done by communications to a single "legal" person who is legally located at a particular place in a single state, so the law of the state in which your employer is located (Washington State, U.S.A.) would govern, although there would also be an argument to looking to the law of the place where the person making the statement was located when they made the statement (which, it appears, would also be Washington State in your fact pattern).
When someone in the U.S. communicates something to someone outside the U.S. that does not have the same level of protections for free speech that the U.S. does, there are federal statutes that provide that a judgment obtained there is not enforceable in the United States.
