It's a brittle star, but I'm not sure if it's a Green brittle star. I have two (brown) in my tank that are a great part of my cleanup crew.
"Green Brittle Star of the genus Ophiarachna. This animal is a predatory fish eater, that does indeed do a spiffy janitorial job when small... but grows quickly, and under darkness of night can/does learn to eat aquarium fishes. This species has been documented to arch up in "sleeping caves" of captive fishes and drop down on unsuspecting meals"