I have found three of these so far. one on the back of a rock, one on the back of a powerhead, and i just found another on the back wall of the tank....
They are called brittle stars. Good citizens. The only exception is one that looks like those and is all green. Detritivores, they also do a good job.
Definetly keepers.
Those look great. I wish I would have those as my main clean-up instead of the bristleworms (they give my wife the creeps). Next time I'm in NYC I would appreciate if you can trade me a few of those.