heya! been there done that.
it suposedly does not 'spread' or at least 'spread fast' so you have that going for you. if you have more of that zo and/or it's not one of your favorites or something, then get rid of the rock.
otherwise, try to break off some mat or polyps that aren't close to the white fungus yet and start those off as a new frag. and of course get rid of the original rock.
when i did this, i pulled the entire rock out of the tank and did the fragging in a bucket outside the tank. didn't want that stuff to knock off loose or anything.
the frags i salvaged from my original 'colony' i put on 2 seperate small rubble rocks seperated from any zo by at least 6". one didn't make it. the other is ok.
a bummer, my 10 polyp 'colony' is down to 1 polyp that while is opening up, isn't looking so hot.
the good news is that all of my other zo frags and colonies are uneffected by it. this was several weeks ago.
gl!