I'd first dip with iodine again. Then in the container with iodine, cut the diseased area off, you'll have to get a bit of clean healthy tissue as well, just to be sure you remove all of the diseased tissue. It might be too late for it though, especially if whatever is doing this has already invaded the rest of the tissue. Once done, place it in low flow, and low light for a few days while it heals.