Asterina star. Really common hitch hikers. You'll see more at night since they are nocturnal. Some people say they are reef safe and just eat coralline and film algae off the glass. I have had bad experiences with them and have found them devouring previously healthy corals in my tank. My tank is very low in algae though, so I think if they run out of their primary food source (algae), they end up turning to eating corals as a last resort.