Generally, in a trial (except a bench trial), a judge is exclusively a "trier of law", which means that they concern themselves with running the trial in a legal matter, and resolving disputes of law (such as if specific pieces of evidence should be committed to record).
The jury, as "triers of fact" are supposed to form conclusions as to who is trustworthy on what issues and to what extent. They are not supposed to confer until they hear all the evidence (and are sequestered for deliberation) so that each juror can finalize their opinions independently.
In a custody hearing, the judge also acts a "trier of fact"; thus, they take on a role of deciding who is trustworthy; specifically, in a custody hearing, their role is to decide who is most trustworthy/best able to care for a dependent child.
Now, stating "I trust Janice" out of the blue is odd, but I suspect that is not what happened. Rather, it was said in response to something that was said or asserted (e.g. "Janice is lying in her testimony"/"Janice asserts something" -> "I trust Janice").
This is not judicial misconduct; nor is it bias, which is "the predisposition ... against or in favor of one of the parties"; disposition due to evidence (such as testimony) is not bias, it is the point of accepting (and seeking out) such evidence.