The green brittlestar is a KNOWN predator in the wild.
It may or may not display this behavior in captivity but it is for sure a risk. I have never experienced it but I absolutely believe others have...and I believe that they have experienced this with several other species as well.
Some species do grow very large as well. Some will easily be at least a foot from arm tip to arm tip.
Personally, I suggest spot feeding brittlestars because, although they scavenge, we tend to keep a lot of other clean up crew around. Brittlestars are highly opportunistic. If hungry, many can and will use various ways to get food, including eating other tank inhabitants. I "spot feed" by making sure that I feed shrimp pellets, etc and that the brittlestars DO get some. But I am always prepared for any to take out the shrimp or small fish. It is a definite risk with keeping them. Remember, there are well over 2000 species of brittlestars and not all of them behave exactly the same way.