It's an asterina.
Does it look something like this?
Notice the little guy on the left looks to have only 6 legs. The number is irrelavent.
There are a variety of tiny sea stars in the genus Asterina that all look more-or-less the same but in practice they may have very different habits in aquaria. But, I figure if they all look the same and some are good and some are bad.... well, they then all end up in "Renee's Bad Category". I'm not sure if some species are obligatory coral predators or if those that prefer corals will eat detrius and algae when placed in a coral-free environment.