Yep. Those green serpent stars will make a "tent" out of themselves in top of a cave. When the fish swims under it attacks and eats them. They are known fish eaters. The hairy brown brittle stars are better additions to a reef then the green serpents. Looks like yours is a beast!
Yeah, I had it in the sump of my old system before I decided to take it down. It will end up going to a different home soon due to the new sump doesn't have a refugium. It's also very quick!