There are many different kinds of asterinas, and the more common ones are algae eaters that hang out on the glass. But there are definitely different kinds that munch on coral. The best way to tell is if the asterinas you see are just hanging out on the glass or rocks, then they are the good kind. However, if you ever notice small white asterina-like sea stars always hanging out on your coral and polyps, then you should be worried. That's what xtalreef says, so i would go by that since they have experiance in these hitchikers.