Yup. That's shell rot alright.
Clean it well with antibacterial soap, rinse. Allow to dry thoroughly then apply athlete foot cream. You may need to do this for about two weeks. In the meantime, change out the substrate and keep it drier. Sitting on wet urine/poop soaked substrate is usually what causes it. If you can figure out how to wet the underneath substrate while keeping the top layer dry, that would be the way to go.