I just now came across this thread and had to post to it. After a couple of weeks of trying to catch him I just banished my Brittle Star to the sump tank! (my makeshift refugium) The little bugger got big and has eaten a total of 2 cleaner shrimp, 1 gold coral banded shrimp (from a mated pair) and 5 peppermint shrimp! And no, he's not a green star either.
I know it was him because he's the only thing in the tank that could eat anything as big as the shrimp. And they just completely disappear, no bits, no pieces, nothing left of them at all! I was feeding him half a raw store bought shrimp every day and after I missed a feeding a cleaner disappeared. He wasn't a suspect at that time (yet) but after I added the peppermints and only saw one the next day and never saw it or any of the others ever again I got suspicious.
I never lost any other critters although I don't know why he didn't get my half blind Yellow Headed Jawfish (he's another story) or my burrowing Zebra Barred Dartfish. Maybe they were just a little to fast for him. I still have one gold shrimp and have more peppermints on the way. Everything has been peaceful for quite awhile now so lets see if it stays that way now.