Redfoots are more prone to shell rot. This could possibly be a start of it. They need a higher humidity but without being on a wet substrate all the time. Keep the top layer dry, while keeping the under layers moist. You can do this by letting the top dry out and pour warm water into the corners of the enclosure to allow the underneath get damp. Other then possibly having to change what I mentioned above, I don't know if you should do anything else at this point. See what others say that has had experience with shell rot.
It's called Scud. I live in S Florida and my Redfoot tortoises get it all the time. It is a type of shell fungus caused by the soil. It is very easy to take care of. Athletes foot cream will remedy that in no time. It really has no adverse affect on the tortoise in that early stage. I actually just had a Redfoot hatch out of the ground and the shell looked almost abnormal. 2 days if treatment and it looks good as new!. If I am feeling ambitious, I will post some before and after pics.
It does look like shell rot. My tortoise had it and in a couple of days it was gone. It was amazing. If I were you I'd start treating him for shell rot
Shell rot, from being too wet. They need humidity, but not to be completely wet. Does he like to sit in his water bowl for long periods of time? The top layer of substrate should be dryer then the bottom layer of substrate...