I'm by no means a zoo expert like pthers on this forum, but I'd say from the picture that you have a fungus of some sort growing on the colony. I can't really tell from the picture how extensive it is.
To be safe you might want to seriously consider removing the rock and cutting whatever healthy polyps you can out of the center of the unaffected area. The removed polyps can be mounted to new rocks and placed in a quarantine tank if you have one available. I would discard the affected rock, just to be on the safe side.
An alternative might be scraping the affected areas down to bare rock leaving the unaffected polyps. You might want to remove a row or two of polyps that still look healthy just to be sure that you got it. Then dip the entire rock in the zoo dip solution listed at the top of this forum or a strong solution of coral disinfectant and tank water. I'd do this in a bucket with some circulation for about 20 to 30 minutes before returning the rock to the tank.
Do you have any other expensive colonies of zoos in this tank that might be at risk if the fungus spreads? If so, you might want to weigh the risk versus the number of polyps left to save. Discarding the rock might be your safest option if it might put a lot of other zoos in jeopardy to possibly save just a few polyps.
I'm sorry that popped up in your tank. If it is a fungus, I've seen it in a few dealers tanks and the colonies were usually discarded if it was widespread. If it was me, I'd try to save a few polyps, remount them, dip them, discard the affected stone, and keep my fingers crossed. I'll keep mine crossed for ya.
