If its above skin level, it may be some type of isopod. I'd try to just remove it with tweezers. I've done this with several mystery parasites. Hold him with a wet towel and be sure you don't do surgery over the tank---in case this thing explodes and releases more 'what-ever-the-hell-it-is". There are a lot of mystery parasites removed by divers, shippers, wholesalers, etc.