Something cheap and huge... and kind of swims slow. Only thing I can think of is a giant frogfish or lionfish!
Did you use the stinky shrimp method to bait the trap?
Thought about interceptor? It'll nuke your amphipod and copepod population though... as well as your cleanup crew. Actually, I ran across one article that observed molts from cleanup crew being used as a food source so you might even want to remove you CUC.