Looks like a species of asterina star. Good or bad? Yes and no. Some depends on what's in the tank. Some species are just algae/detrius grazers but there is one that may feed on the tissue of sps
corals. Figuring out if you have that species could be alot trickier and may just take trial and error. They also reproduce quickly. As with any nuisance though, limiting their food source should help with that.
Here is a profile I did on them but I would also suggest doing a google search to see what you come up with. HTH