That is a common fungal infection. It is often caused by tortoises sitting in the same damp bedding that has accumulated a lot of urine over time. It is also contageous and may be from another tortoise sharing the same sleeping quaters that may have the fungus already.
Sometimes wild caught redfoots and yellowfoots that are from really wet areas, or that have been held in dirty compounds before shipping will have it as well.
The good news is that it is relatively easy to get rid of.
Clean the spots by a light scrubbing with soap and allow to dry completely. Then appy one of the anti-fungal creams sold for athlete's foot or jock itch. It should clear up pretty soon as long as the bedding or sleeping area is also cleaned or moved.
If left unchecked, it can spread to the carapace and become very hard to eliminate.