Yes, it is an Asterina sp starfish. Most species of Asterina don't reach plague level unless the tank has a nutrient problem (and thus algal growth), but this particular Asterina specie that you have with the darker coloration, any that have numerous or disproportionate legs, and those that are larger than usual seem to often be problematic. I personally question if this particular specie is reef-safe or not...I question if they might be partial to coral appetizers on occasion. This particular specie of Asterina I would remove from the tank. The tiny, plain white ones seem to be quite reef-safe.
http://www.garf.org/Star/starfish.html