I made this post over on Reddit and they said I should come over here and ask you all. Sorry to be lazy about just posting the link but I am also travelling while trying to help solve this problem. Thanks in advance.
I agree with all of the others. Its either some sort of abrasion or rubbing from something rough in the enclosure, or some animal is chewing on it.
Someone on the thread quoted my exact words from a thread here: Treat once with betadine or peroxide for disinfection, and then use Vetricyn on it daily until it heals.
It should not be living with other animals, as it will eat their poop and they could injure each other. I've had kangaroos for decades. They can bite and chew like rodents, they can kick, and they can be quite territorial at times.
This tortoise also needs a heated night box/shelter. I doubt the nights there are staying above 80, are they?
Illinois tortoises need a heated shed for the cold winters and for the temps we are having now, cold! He should not be living with anything but himself, get him his own space asap!