I was going to say to pull the rock out of the system and pull the corals off fom the base until you mentioned the sponge. This isn't the best thing for your tank, but what I'd do is literally rip them off the rocks as close to the base as possible. If you do this in the tank, be ready to do a water change and run some carbon. Softies love to release crap into the water, so the WC and carbon will help reduce any effects. The mothers you pull off the rock will look unhappy for a while (up to a couple of weeks, maybe), but they generally do okay with this kind of treatment.
Depending on the type of rock, you might be able to take some bone shears to the rock just below the base of the coral and break off pieces of the rock to remove the mother colony. That would be the least detrimental to water chemistry. The mothers will generally grow back from the pieces left from the base when manually removed.