Take the rock out. Get as much off as you can with wire cutters, bone cutters, wire brush or whatever you have. Wear eye protection just in case (good habit to be in). Put it back in a bucket of water for a while so the other "good" coral can catch a breath. Take it back out and towel off the bad areas so that they are decently dry (not soaking wet). Sprinkle dry kalk over the bad areas until it is coated like a powdered donut - you can also mix up a really thick paste of kalk (like toothpaste consistency) and wipe it on with a brush. Let this sit for 5-10 minute to harden or cure - it does not really cure, but you know what I mean. Put it back in the tank with all of the flow pumps turned off so that the kalk mixture does not blow around - leave the return pump on. The next morning you should be safe to turn the flow pumps back on. For really stubborn stuff, you might have to do this more than once - don't wait too long if you see it coming back.
Other than that, break the good corals off the rock, mount them somewhere else and "cook" the nasty rock in a bucket of saltwater for a few months.