It's an asterinid starfish. For most people, they are harmless, though some have implicated them in more dubious acts. I, personally, have never had an issue with them or seen any issue resulting from their presence.
The only issue I have w/ these is that they may eat at coraline algae.
I've had times in the past where I've seen patches of coraline eaten away, and I love the look of coraline, so I added a harlequin shrimp to eradicate them.
Only thing is now I have to supplement the harly w/ chip stars, since they can wipe out asterinas pretty fast, and they only eat starfish.
I actually think the coralline may be incidental. I always see them eating animals, be it worms, 'pods, etc. Of course, whatever is underneath them when they are eating one of those things is fair game, like coralline.
About the diameter of a dime, but most will split in half before then, creating more. They usually plane off after a while, so most won't experience anything like plague proportions. Some do, but you won't see it often.
I have one of those, i find it fun to watch, with all its little legs and feet. It once hitched a ride on a turbo snail, thats the worst its dont in my tank.