[Myka]

Soak it in vinegar for an hour with no circulation. The vinegar will kill of the outside layer of the rock, but with no circulation very little vinegar will make it into the rock. After the vinegar bath put it in a bin to cycle and check that nothing grows back. You might have to do the vinegar bath a couple times. If it was summertime I would suggest using a powerwasher on the rock to blast the corals off before the vinegar bath.