asterina star. some people say they eat sps, but i never had any problems with mine. they split alot and soon you will have tons. looking at my 15 with the lights out i see about 30 on the glass.
I had one, went missing and surfaced after 3 months, and in one weeks time disappeared....never to be seen again....(I don't think it caused any problems).
i had one on my live rock, and lost it for a month, borke my tank down, left the rock out for 6 hrs, put tjhe rock back in, and saw the star the next day.....