You mean the asterina star? They are generally considered harmless. Film algae eaters. There are loose and largely unsubstantiated reports of certain varieties (mostly the darker blue ones) munching on soft corals, but to the best of my knowledge, this has never been proven.
Some people do not like them simply because they breed like crazy. I tend to leave them alone because I've never seen them hurt anything.
Someone stop me! Please! But... not just yet...
55 Gal Tropical FW
55 Gal Mixed Reef