yup, that's an asterina starfish but the jury is still out on that one. I did some research on them and while the articles I found say that it will eat some types of corals, Calfo and Fenner's Reef Inverts book says that it more than likely is grazing on necrotic tissue on an already diseased or dying coral. It is however a very prolific species and can rapidly multiply. they are great algae and detritus eaters. keep an eye on it. I've got a couple and so far have had no problems.