Wow really using athletes foot cream works on a tortoise shell? Would never of even thought of that at all. But good to know.I'm by no means an expert, but that looks like shell rot to me.
Try treating it with athlete's foot cream daily 7-10 days after cleaning the shell with warm water and a rag or toothbrush ... I bet you'll see an improvement.
I rub a small amount of a homemade lotion on my tortoise's carapace and plastron once a week, which may or may not help his shell, but I'm positive the weekly inspection is a good thing.
Good luck and congrats on your new tort!
Jamie