I'm assuming you're worried about employment contract clauses that specify that anything you do while working for the company are property of the company. Usually they say something along the lines that you agree to give the copyright and ownership of anything you create, on or off the clock, to the company, so I'm going to assume that's what yours says. This clause is to prevent a case where an employee claims they invented/came up with software they wrote for the company off the clock, but wrote on the clock, which would leave the question of the owner of the copyright of the company software vulnerable to lawsuits or other legal trouble.
That said, these are also notoriously difficult to enforce for side projects or side hustles. Even if the company had your git logs proving you wrote the code while working for the company and so the code was property of the company, you could claim that you came up with the idea and sketched it out on a napkin before you started working for the company. Trying to battle you for it in court would be unlikely to be worth the money and PR risk for the company unless you had invented the next Facebook.
I have not seen or heard of a company that wasn't operating in good faith with regards to enforcing this clause of the contract. Most companies understand that software developers will want to freely develop in the free time without harassment, and the companies are mostly worried about disgruntled employees claiming copyright or ownership of company software in the future to file lawsuits or block mergers (in which case, they can pull out your signed contract and immediately prove you signed away your rights). I would not worry about mentioning it unless you are seriously considering forming a business, in which case you're more likely to run afoul of clauses preventing you from working for other companies while employed for them. Maybe double-check on Glassdoor to make sure there aren't complaints of the company being a stickler for enforcement.
