I'm not quite sure what we are supposed to be looking at in the photo. It looks like an area of wetness and then up above that there's an area of white. I don't see anything wrong with the shell. Maybe you could take a picture with your finger pointing at the area of your concern.
Well, obviously none of us has an answer for you, sorry. If it is of great concern to you, maybe you should schedule a vet visit. But don't allow the vet to give the tortoise a vitamin injection.