Which sea star are we talking about? Mini bristle stars? Asterinas? If you know which one it is, please let us know. If not, try finding it on here:
and then let us know which one you've found.
Generally, i would say you probably got an Asterina or a Bristle star, of which both or harmless to coral. Their have been few testimonies about asterinas munching on polyps, but very few, and not to mention that the polyps began disappearing when there were giant hoards of the asterinas-and even then when people had tons of them no corals were really harmed. There's a few people here that have tons of asterinas in there reefs and have never had a problem with them.