Fishy's right - one of the many small species in the family Asterinidae that reproduce by splitting in half. Most of them are harmless micro-grazers but some of the species are predators that eat corals. The predators show up much less frequently than the harmless ones. You'll need to watch that one to make sure it's a grazer.
Asterinids can be beneficial cleaners. The real problem is that they tend to reproduce very rapidly since tanks normally don't have predators which can keep the population under control. People have posted pics of their tanks just covered with hundreds of visible ones, which means many more hidden ones in the rocks. With that many hungry mouths they may start tasting everything in the tank. At that point they're there to stay. You can scrape/siphon/get harlequins to get rid of the majority but there will always be a few in hard to reach places that can start the population explosion all over again. So as much as I like animals everyone else considers pests I suggest getting rid of it.