Certain ones definitely eat zoas and at the very least, tick off palys. I've had entire colonies decimated by them (I've watched it happen). From what I have read and been told, certain ones do this while the vast majority are benign. I don't know why, but the ones I have are particularly fond of tubs, rastas, and pretty much any of the brighter and more colorful zoas. They tend to stay of the green and earth tone varieties. I picked up a harlequin to go after them, which he has, but stays on only one side of the tank which is opposite where I really have the problem. The harlequin began eating immediately and has grown considerably.