Don't move the rock, move the crab.
After he clears that rock of polyps he might move on to another! Maybe most crabs won't do this, but one that will isn't worth stressing about.

I had an emerald crab do the same thing. He'd snip off a polyp, nibble on the bottom, decide he didn't like it and let it float away. Then he'd sample another! He went back to the store, and I've been hesitant to try any more emeralds since then.