If it's just one little spot it could just be a discoloration. But pictures are always helpful. I'm dealing with a ball rescue right now that has a pretty decent case of rot. And by decent I mean not terrible but obviously not good. I've dealt with a few small cases before with other rescues and it's actually quite easy to deal with. Grab a bottle of betadyne from Any drug store it's an iodine 10% solution and do betadyne dilute soaks in water temp about 85-88 1x daily. Keep the enclosure as clean and as dry as possible you still have to give humidity, but try and not make it overly wet. I use either paper towel or newspaper with a rot case. Change the paper towel or newspaper daily. Anyways you keep doing this daily until it starts clearing up. Again I don't keep corns but I have dealt with a few rot cases of differing severity and this is what the vet taught me to do back on the first one. Worked well. This sounds like a skin spot though like a discolored scale. I've included a couple pics of the rot I'm dealing with now with the rescue. Hopefully you can compare and it will help.
Morgan