Russian tortoise bottom shell white spot

[taropoo]

Hi everyone,

We noticed our Russian Tortoise Clyde has a white spot on his bottom shell, it doesn't look like shell rot and we are not sure what it is, we figured we'll ask the fellow tortoise lovers before we really panic and take him to vet. Thoughts on what it might be? We already tried gentle brush on it but it didn't come off. We are also attaching his top shell just to get a sense of how he is doing.

Thanks,
Bonnie