Sick Butterfly Fish

I would maybe suggest a fresh water dip. use distilled water at about the same tempeture as the tank and put him in there for 3 minutes but if you see any signs of stress you should put him back into the tank IMMEDIETLY, you should do this every 3 days until it goes away. or put him in a quarantine tank

Have you tried identifying it? I can think of a few ailments that might cause growths, none which will be effectively treated with a SW dip, some which don't have a good treatment option available (viruses). Here's a decent page: http://www.fishpalace.org/Disease.html