Is my statement about a mosque imam, "He doesn't speak much with people," considered backbiting (gheebah) or slander (buhtan), and how can I absolve myself of that?

Backbiting is to mention your brother with what he dislikes, and slander is to mention him with what is not in him, and it is worse than backbiting. If you have backbited or slandered someone, you must repent and ask for their forgiveness in general terms without mentioning details. If you fear corruption, then mention them in a good light, seek forgiveness for them, and supplicate for them. You must also command anyone who heard the backbiting with you to repent. It would have been better to advise the Imam directly instead of backbiting.