It all depends on the specific kind, and it's virtually impossible to tell them apart. In 6 years & 6 tanks I've never had the kind that eat anything but film algae & coralline. That's where paying attention to what's going on in your tank comes in. Mine cruise all over my zoas, and I've watched them sit on a closed one for a while, then when they get through cleaning off the algae & leave the zoa opens back up. Now if they leave and the zoa is damaged, pull them. Or, as I said, you can pull them now just in case. The coral eating kind are rare in home aquaria, but it happens, especially when people share frags or a store sells frags with the wrong kind on them. That's why you usually hear about the bad ones in spurts instead of all the time. Same thing with the coral eating amphipods or isopods.
