I am not an expert by any stretch but I thought of a couple things..

There is about an inch of clay balls on the bottom that is covered by coconut substrate. There is enough water in the bottom to cover the clay balls but not the substrate..
Do you have a way to remove the water from the drainage layer or is it just sitting stagnant covered in coco fibre? Hard to tell from the pic where the water level is sitting or how damp the soil is. Is there a mesh layer between the balls and dirt to keep it from falling into the drink?

Agree that water wicking could be a concern but tank is fairly new, correct?

I feed them once every two days. I take them out of the tank and put them in a plastic tote and they eat about 2 large crickets.
I imagine they could be fed without being removed and causing unecessary stress but that's just an opinion. I'm definitely not an expert on gray treefrogs. Maybe more branches and some plants to make their home feel more "homey" and keep them a little cooler as mentioned already. They can survive the colder months just fine it appears.

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