I only notice them eating zoos that are weak and closed up, that is they have some algae growing on or around them. I think that they target the algae and then end up eating part of the zoanthid, usually killing the zoo which wasn't too healthy to begin with, but might have survived if they weren't chomped on. I'm going to try to handpick out as much as I can and throw them in my sump to propogate, then get a harlequin to clean up the rest in the display. That way the harlequin won't run out of food either.