It’s very hot and dry in Riyadh. Imho, the affected area is right around the shell edge where the other Bully tort just contines to rub, bang and bully. Those sharp edges of a tort’s shell can easily cause that raw rough damaged bloody area. I don’t think a shell fungus looks “bloody” and bruised.
Of course I realize the climate factor and it raises the question. However it looks to me more like eruptions rather than impactions. Still and of course the separation is a wise idea either way.