Never had a problem with them eating anything. You will find them on corals, etc. but only when there is a dead spot for them to feed off. I have never seen them eat at any living tissue before. Some people say they can be bad, primarily the 6-legged ones, but, I don't think that is the case. They are one of the first ones to feed off dead coral tissue so people may perceive that as them actually causing the dead tissue. Harlequin Shrimp will definitely eat them but as Poseidon mentioned you must move them once they are gone or buy starfish for them to eat.
I found I had tons of them for a while then they slowly start to disappear. My guess is they overpopulate and begin to die off from lack of food.