That's an old injury. At one time prior to you getting him, he was bumped on that spot and the outer layer of keratin was skinned off. It's nothing to worry about. You see it a lot in russian tortoises because of the harsh conditions they endure being captured then contained for shipping.
Good catch R/S/T. From the background in the pictures it looks like you can see the bottom of the enclosure through the substrate. Russian tortoises are diggers. They're most comfortable in substrate that's at least 4" deep.
Hello his substrate is almost 4" I'll add some more coir, thanks for the tip! That grey is the corner of his enclosure that has a piece of slate stone rock. I have slabs of rock around his water dish and where he basks.
Hello his substrate is a was just the corner of his enclosed that has a piece of slate rock. I have slabs or rock around his water dish and where I place his food dish when its food time.