That's an asterina starfish. They are very common hitch hikers. 99% of the time, they will just eat film algae off the rocks and glass and are harmless. But sometimes (especially if your tank doesn't have a lot of algae), they will eat corals. I had them in my tank for years and never had any issues with them. Then, all of a sudden, I caught them eating my zoas. It's up to you if you want to keep it or not!