Hey,
From what I know (which is I admit is minimal...) they are good for the most part-excelent cleaners. But sometimes they get a taste for corals, in which case they come out. I caught one of mine red-handed(?

) trying to munch on my Hammer Coral.... been trying to catch him ever since. My LFS told me that unless you do catch them, leave them. Once you catch them, get them out. Makes sense to me. (my bristle worm is also suspect in my dead torch coral, but never caught him on that one. )
S.