Firstly, a little mold will not hurt them. I would bump the humidity back up to 45-50%, especially when you do not have much substrate to maintain it. Get the temps to about 70 and maintain them there. I know they can survive winter, but they'll digest the best with a higher temp The 12x12x18 will work while they are juveniles, but you're going to want to get something bigger as they grow. That's not enough space for two grown grey tree frogs. If he can't climb, I would get rid of the reptile carpet, you do not want it to scratch his sensitive skin. Use damp, unprinted paper towels, changed every 1-2 days. The water needs to be treated even if it is well water. Helps with the heavy metals. That could be the issue if he's been soaking a little extra and the water isn't treated. Does the stomach feel hard? Is it squishy like it's filled with air or with fluid? Are they big enough to take small crickets yet?

UV is not necessary, the calcium supplements will work just fine. You should also get vitamin supplements, fruit flies aren't exactly packed with nutrition, the sooner the better!

You may want to get the fecals tested to make sure there are no parasites causing issues.

EDIT: And welcome to the forum! Greys can be really great pets!

I'd also get a more reliable way of measuring temp/humidity. You can just use a plain glass floating thermometer (from the fish section, $1-2) to measure the temps. May want to get a digital hygrometer for measuring the humidity. Not much in the way of substrate and heat lamps could dry things out fast.