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.
I donot have direct experience treating a tortoise for a fungal issue, but when I handled a fungal treatment for my dog, I made sure he never had the chance to lick the ointment.
If I were you, this is what I would do:
1. The best time is overnight. My Sulcata usually finds his sleeping spot around 4-5 PM and gets active again around 8-9 AM. I think your Sulcata must have similar routine. You can use this habit to your advantage: After 4 PM, unplug the soaking area so the water drains. Clean it up a bit: remove any poop, urates, or leftover food, flush, and let it dry. After that, apply the cream to the bottom of the shell and any spots on his legs that don't look good. This gives the medication at least 12 uninterrupted hours to fully absorb without being washed off. When 8 AM rolls around and the sun starts to warm things up, put him in the empty soaking area. Gently brush him over with a sponge and flowing water (I used baby soap when mine was a hatchling, but since he is older now, I just use water). Flush him with plenty of water, remove him from the area, flush the soaking area clean, and then plug it back up so he has soaking water until 4 PM. Do this daily until the shell, hand and leg look better.
2. Regarding keeping a concrete hiding place dry. Since the enclosure was built for them, you should ask your cousin if the hiding place actually stays dry (honesty matters here!). Normally, when you build an enclosure like your cousing, you plan for the hide to be at the highest point and the soaking area at the lowest so water and pee flow away. If that wasn't done, I highly suggest using a rubber mat inside the hiding place. That way, when he sleeps, his shell and feet aren't resting on wet concrete.
3. Regarding the disinfectant. Again, I don't have experience cleaning my tortoise enclosure with chemical disinfectants. Does anyone else have experience with which disinfectant is safest for a tortoise? If it were me, I would do a deeper clean once a week using 3% Hydrogen Peroxide, similar to how I clean my bathroom. Make sure the tortoise is NOT inside the enclosure while you do this! Spray a standard 3% hydrogen peroxide solution directly onto the concrete floor and walls. Let it sit for 10 to 15 minutes. The bubbling action actually helps lift dirt and fungal spores out of the surface. Flush it thoroughly with clean water and let it air dry completely before letting him back in. (Note: I am not sure if the hydrogen peroxide will fade or discolor the green paint over time since it acts like a mild bleach, so he might want to test it on a small corner first!)
I know this seems like a lot to do, but I hope your cousin is patient enough to care for him. Regarding point 3, it is definitely better to check with others on how to safely disinfect. Your area is more humid than mine and also gets rain more often, so keeping things dry will be extra important.
