They are a good part of your CUC. We used to pull them when they got "too" big though. Get something that eats them, will help control them. We got an arrow crab, which was cool. I would find him the big ones with tweezers and toss them to him. He would catch them and cut them in half and eat them. Several fish species will eat them too.

I have heard people say they can irritate corals, but we didn't experience that. Doubtful they caused the demise of your sea hare, but they probably helped clean him up once he was dead. Just my opinion and we all know what that is worth... 2 cents.