Moss is a medium that can be used in the moist hide. If you are talking about as a substrate for the tank, then no, it is not suitable for the gecko's habitat.
Use moss or paper towel in moist hide. For substrate/bedding, I personally use paper towel because it's more easier to clean. Or u can use reptile carpet. Moist hide is to help them shed alot easier n for females, that's where they lay eggs. Mist the moist hide once a day to keep it moist.
Substrate = the stuff you put on the floor of the tank. A lot of people use tile, paper towel or reptile carpet. I'm new to this as well, from what I've been told/read it seems paper towel is good if you just want something easy to clean, if you want something that looks nicer slate tile is the way to go. I was going to originally use reptile carpet but a member of this forum actually advised me against it, read up on it a bit more and found out the tile stays a bit cleaner than the carpet.
you need both a DRY hide - one with nothing in it, and a WET hide - one with something in it to keep it moist. The moss you were referring to could be used in the WET hide. You wouldn't put moss all over the tank, the tank should actually be very dry with the exception of the wet hide, and the water bowl.
You also need heating of some sort, I just set up mine with an under-tank heater made by zoomed. eco-terra makes one too. They sell them based on tank gallon size. You need to keep the hot side of the tank around low 90's on the floor.
Those are pretty much the major things you need for the inside of your tank, everything else is pretty much decorations, stuff for them to climb on, etc.
Your tank looks great so far, no loose substrate, three hides, I'm assuming a UTH underneath the enclosure, sounds good to me so far.
You can use misted moss for a humidity hide to help keep it, well... humid. When placed on the warm side of the tank, the combination of moisture from the moss and heat from the UTH will create a mini humid environment, which can help keep the geckos hydrated (I often see mine lick at the water droplets inside) and provide a big aid with shedding of the skin.
This humid hide can be made out of a simple Tupperware container with a hole cut in it (watch out for sharp edges!) and filled with moss or damp paper towels. I personally use paper towels because of their easiness.