Like I said, nothing in 6 years or so has ever eaten this stuff, not even a lawnmower blenny, no snails, no hermit crabs, no emerald crab and the list goes on and on. I've used toothbrushes, screwdrivers, knives and all sorts of other tools to try to kill it... nothing.
I would suggest simply getting that rock out of there. Put it back in next year if the stuff ever dies.