One thing I was told about Moss and some Substrates is to bake or microwave them to kill off any mites or mite eggs that could be hidden inside.
As for keeping humidity up I place clumps in all 4 corners and along the back side of the water dish up against the glass. Also during Shedding I put clumps in both hides. You can also try putting in a larger waterdish if you have the room. I did and it raised it up about 6-8% but it was still kinda low. Now I am at a comfy 62%.