Yeah, Tony, the green ones are great scavengers when they're small, but once they get large, they'll eventually develop a taste for fresher meat...
Can't find the video now to save my life, but years ago saw a video of one hunting. It'd stand up, forming a fake "cave" with it's legs and body. A small fish would swim into the safety of this "cave" and the starfish would drop down while twisting to close the "bars" of the prison, drop down on the fish, kill and eat it...
However, only the green ones are known for doing this. If you have any small fish in the tank with a large green brittle, I'd highly recommend ya' find a new home for either the star or the small fish...