Looks like asterina starfish. More than likely if you have one, there are more. I'm indifferent to them personally as I've not had a bad experience with them. A lot of people will say pull them as they may eat softies. But again, I've never had the issue. In laymans explanation I've heard as well the lighter ones are ok, but the darker seem to munch zoas and the like. I've got the crisp white ones and never experienced the other so I can't offer first hand knowledge on them. Hope this helps a push to the right direction for whatever action you may chose.