does razor blade work? go check out the zoa pox discussion, most people to take any carbon out, do water changes and put in higher flow area and wait it out. sounds like more common when weather gets colder, kinda like a 'zoa cold/flu,' though recent discussions around furazone-2 being a treatment..
if you want to take out the entire polyp or colony that may work, though if it is some kind of virus/fungus, unless its on a recent addition to your tank, most likely its in the water and the other zoas have already been exposed.