It most likely has some sort of infection, they don't describe anything like this on the site that I went to, but you can definitely call a vet or a pet shop and see what they tell you it might be. Then from there, you could take it to the vet or maybe the pet shop will have some sort of medicine that can help. If you have a Petco in your area, I would start there. Good luck.
Actully my step cousin has 1 pet turtle he use to have 2 but the boy died and I think it was because of his girl turtle that had pink on the bottom and I think it was an infection..so I am wondering the same thing....