Hi Nero~
Firstly, you need to get those walnut shells out of there. They will absoultly cause and impaction. You can use the following for a substrate: Repta carpet (looks like astro turf) Slate tiles (these are best for traction), other tiles, Paper towles. And if he is over 7 months you can use Washed play sand (NO OTHER SAND). If you use sand, do not dump crickets in to the tank to run around because then he can kick up the sand and ingest it. keep all food (including crickets) in dishes that can not get sand in it.
Second: Your temps need to be 100-105 in the basking area. The center of the tank should be around 90-98 and the cool end should be 75-80 deg. It sounds and sorta looks like you are growing mold in the tank. Get a humidity meter. The humity should be no higher than 70. You want it to be between 40 and 60.
Third: you need to be using a UVB lighting system such as MVB (Mercury vapor bulb or a repta sun 10.0 (not to be confussed with repta glo 10.0) Do not use coil lighting this can cause blindness. The lighting you have is not UVB which means eventully your guy can get medibolic bone deases from lack of Cal D3 defficency.
Once you get the temps regulated along with the humity this mold shoudl strat to go away. You can try using a medicated dissinfectant. I have heard of a certain type that is used for reptiles, but some one else will need to help me out with them name. (You can just use anything of course.)
I hope this helps!
Stacey