From what you have said, it seems as though your tortoise had been sitting in the water pool for a very long time and algae grew on its shell and even on some of the leg scales. I think your cousin is describing the algae growth as "moss".
Do you believe the "pool" was always kept as clean as in the photo you posted? I don't believe it was kept that clean, and I think that allowed algae to grow on the tortoise and that is how this shell damage occurred.
But he said sulcata not sleeping on the pool all day. Just pooping and bask at same time on that pool then roaming on the enclosure..i will ask my cousin to start buying those athlete foot cream and hoping it will be better
As Zovick mentioned, the most important thing right now is to start a fungal treatment.
Get an antifungal cream ASAP. You can find generic Miconazole or Ketoconazole cream on Shopee (1 tube is 5gr/10gr), it's much cheaper than the branded ones, works exactly the same, and you'll need a lot of it for a shell that big and several days application.
(@zovick — could you advise if Miconazole or Ketoconazole 2% can be used and safe for the tortoise's shell?)
Here, I think how you should handle the treatment and the enclosure:
Safety: Always apply the cream after his soaking time. Important: The ointment might not be safe if swallowed. Before his next soak, make sure to thoroughly flush his shell to remove any leftover cream so he doesn't drink contaminated water.
Fix the Moisture: He must have a completely dry hiding place. Make sure the hide is elevated so the rain never gets inside. If the hide stays constantly wet, the fungus will never go away.
Disinfect the Enclosure: Talk to your cousin nicely and ask him(or diy) to deep-clean the enclosure. Spray a disinfectant, leave it for 15-30 minutes, then flush it completely clean with water. This will kill any invisible algae or fungal spores left behind.
