What I would do is to paint the overall area of the moss using alternate glazes of a yellow and blue (assuming a greenish moss), building up the areas of light and dark. Then mask off the surrounding area (either with masking fluid or frisket, a piece of masking plastic, or a piece of paper) and use an old (clean) toothbrush or battered brush to spatter the moss area with droplets of paint.
To get paint on the toothbrush, you can either dip it directly into some paint, or put some paint on a brush and then run this over the toothbrush. The latter method gives you more control over how much paint there is on the toothbrush and how it's spread.
You don't want the drops to be too uniform in size, nor have large blobs of paint. Practice will teach you how hard/fast to run your finger (or a brush handle) across the bristles of the toothbrush and how far you should be from the paper. Once the desired dots are in place, and dry, I'd then glaze over this with a thin layer of yellow or blue, to unify it all.
A dry sponge dipped in a little paint and then dapped on the paper will also give a suitable effect. Or, if you've lots of patience, you could also use the tip of a fine brush to stipple on the paint, building up layers of small dots.
I'd suggest trying a few practice pieces first, before doing it on the 'real' painting. Try doing it with different colors when the paint is wet, and leaving each color to dry before applying a new one.
You may not achieve what you want the first time you try that subject (or even the second, third, fourth time), but every attempt will take you a step closer.
