A couple of days ago my tortoise of aprox 4 years woke up from hibernation. One day after feeding it in my garden y noticed that he had a large stain in his shell. Im not sure if its shell rot, lack of calcium or something else. Aside from that he looks completely fine.
It looks like he was allowed to sit either in water or moist substrate during brumation. If this were my tortoise, here's what I'd do:
1. Set up a tub with high sides that he can't climb out of and fill it with warm water that comes up to the middle of the tortoise's sides
2. Leave him sitting the water for a half hour, then at the end of the time, take a soft bristle brush and gently scrub the whole tortoise to remove any dirt, paying special attention to the plastron, and the white bits
3. Dry the tortoise well then put some athlete foot cream all over the whole plastron, rubbing in in to help it absorb.
4. Keep the tortoise on newspaper for the rest of the day so the cream has a chance to work before it's scraped off in the yard.
It does look like some sort of fungus possibly. Get some athletes foot cream and apply it once a day for a few days and that might clear it up. It won't hurt anything.