i found this guy on the glass near the top of my tank this morning. My guess is that it might be a nudibrach but I am not sure. I'm really concerned if it's harmful and how many others might be in the tank. It's a 120g mixed reef tank with sump and fuge.
That's an Elsyia. Did you get some macro algae recently?
He's not harmful, unless you really really love macro. Their numbers never really climb even if you have macro and they don't seem to fare well long term in captivity. I've seen many people who have had them, but rarely do they "keep" them and see them long term.