substrate thickness

depends on the moss...

You don't need tons of substrate I sometimes bank mine very little on the hot side and 2-3 inches on the cool. The dirt like stuff you are seeing is likely eco earth. It is a ground coconut husk product, it works quite well and holds humidity better than anything else.

Bah got my tank all set up to.. so im thinking I should go with fine dirt instead " coco husk" and throw some moss in there too.. also it should hold/transfer heat much better since there isn't much void space like with reptiles bark

What type of moss is np friendly?

sphagnum moss is fine. you can get it for like $3 at home depot