Definetly an asterina star. Amcarrig is right, they have been reported to be "not reef safe" but people continue to counter that and say that they eat micro-fauna and algea.
They reproduce rather quickly and will become a plague over time, I would take every star you see out, as there will be plenty of those boogers. You know they drop legs and reproduce just like that. That's why when you see an asterina loitering it will have missing legs.
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30g reef tank, CPR CY192 filter w/ DIY plumb running Rio 17HF return, Coralife 3x 9w UV Sterilizer, 3 24W T5-Helios 10K Daylights/3 24W T5-Helios Blue lights, 2 Logysis blue meteor light strobes (moonlights/24 Blue LEDs).
Tiger tail cuke, asst. snails/hermits, asst. feathers, rainbow acan,zoos,shrooms,bubble,galaxea, asst. shrimp, 2 ocellaris,mandarin,zebrasoma xanthurum,pink-spot watchman,red-striped pistol. | 
baby star find.. identification please.. w/photo 
