Here is the way to make great looking moss that is fully edible:
1) Make a batch of thinned Royal Icing.
2) Add to the icing, Kelly green gel coloring, and a few drops of Yellow gel coloring. Adjust to make a good slightly darker than sage green, color.
3) Prepare your countertop surface with a good coating of pan spray or shortening.
4) Take carpenters putty knife and spread the royal icing onto the counter top surface in a long clean wipe, a thickness of a peice of tissue or crate paper is as thick as you should have after spreading.
5) Allow to dry for a few hours, or use a hairdryer for faster drying.
6) Once the royal icing is completely dry, take the putty knife and begin scraping as though you are cleaning the counter surface. Make sure that the icing is very very dry, or you will have a gummy clumpy mess.
7) Spray the surface of the area that you want to attach the "moss" to, with an edible spray adhesive, and then using your fingers(wear gloves to prevent or to help lessen the chance of melting the icing before you have a chance to place it.) take the moss a bit at a time and dab it onto the adhesive coated surface. Do this in little dabs until you reach the desired effect.
Tip: To make it look more realistic still, airbrush in random strokes and sparingly, with a light brown color. This will add dimension to the moss and make it look as close to the real thing as it can be!
Hope this help! If you need anymore help just ask!
I also put some directions and pictures in this thread: http://www.cakecentral.com/cake-decorating-ftopict-634776-moss.html