I think CUC means Clean Up Crew.
You can keep snails and small crabs, but a young octopus will eat some of them at first, then mostly leave them along.
I wasn't too happy with a pencil urchin (looked nice, but ate my coralline algae) and later found out they are best for 200 gallon tanks and above. Urchins with sharp spines are to be avoided.
A small brittle star is OK.
Nancy