Gnaw marks from a rat?
It wouldn't hurt to place anti-fungal cream on the spot for a few days.
(Athletes foot cream)
It's a very strange spot to have a fungal issue and looks like some other type of injury.
I've only recently taken this Hermann on, he'd only a few weeks previously woken up after a 6 month self hibernation in the previous owners garden after escaping his enclosure! She did show me this mark and neither of us were that concerned because it didn't smell and wasn't spongy.. It still isn't! Just seems to be flaking away now? I'll get the athletes foot cream like you've suggested? Any brand ok?
I buy mine at the "dollar store" because it has the same active ingredient as the $10 ones.
(Same for the $1 antibiotic ointments.)
Two bucks well spent as part of a tortoise first aid kit.
I do hope that someone else chimes in about what this could be.