if you dont have a lot id just do it manually. they come off pretty easy if you like pick them off just make sure you dont break them or leave bits on the rock because i think they can grow back... its pretty easy though. i just plucked mine off with a pair of russians and problem solved. every once in a while i see one now but i just pick it off.
plus its free! in my opinion, the emeralds were pretty useless, except to pick up excess food, which the crabs and snails get anyways.