Do a search on 'Asterina' and see if what comes up looks like what you have.
Uh, here is one page I quickly found .. pics not that great but can be used as a jumping-off point for other searches ..
http://www.wetwebmedia.com/asterinafaqs.htm
Quick summary from me though: the jury is out whether they're good or bad. "Good" because they tend to eat undesirable algaes (hair algae, etc.). "Bad" some say because some may nibble at SPS. With that said, I have hundreds of these types of stars, and tonnes of SPS, and never have I noticed any of them being munched by these stars. So I tend to side with the "they're good" camp. But YMMV.