Mate, I've never heard of "supermoss", but I imagine it's some form of natural lichen or moss.
Personally, I think lichen and moss usually look terrible in dioramas. They're OK for an architect's model because the whole thing is just a visualisation of a proposed design (in short, a fantasy) but in a diorama, or even a model railway layout it almost always just doesn't look convincing enough.
Not so sure about your plan to use green felt, either. Even if you can find a felt that's not too bright or not too dark, I think the colour will be too uniform to look realistic.
Visit the model railroading section of your local hobby store (or look online) to consider some of the Woodland Scenics products. The best product for grass is either static grass, flock, or prefab grass matting, which is available in sheets or rolls. Buy a selection of different colours and mix them to obtain a realistic look. There are plenty of products available to help you make convincing ground cover.
The most important tool to use is your own eyes - look at real grass and real trees and try to simulate them in scale. Same goes for rocks, soil & sand. Note the different textures, colours, shades & structure of the natural item.
As for the white glue; I generally thin it with water at about 50:50, but it depends what I'm trying to glue. Some things need a stickier solution than others.