Bonsai is an illusion and bonsai artists create illusions of grand old trees. Moss, like so many other things can add or distract from that illusion. Too many people fail to realize the purpose of moss, they simply plop it on without thought or inspiration and what they achieve is moss that looks exactly like what it is, moss.
Used correctly, and with the same creative talent that makes a small shrub look like a huge ancient tree, moss can give the illusion of grass, undergrowth, and other naturally occurring plants that add to the overall illusion.
How can moss be used creatively? One trick I like is using a darker shade of moss inside the drip line and a lighter shade out side of it. This gives the illusion of grass being shaded by the foliage, it can make the tree look like a good place to rest, out of the sun, int the shade cast by the magnificent old tree.
Another common mistake is when moss is allow to grow over the entire surface area as mentioned above. I have seen great old scarred and twisted bonsai with such a full moss covering. It always makes me wonder how the grass escaped the extremes of nature that sculpted the tree. I wrote more about this in an article here
http://www.artofbonsai.org/forum/viewtopic.php?t=102.
We style a small tree or shrub to give the illusion of a huge old tree, we should style the moss, if used, to resemble grass, or other such undergrowth, it should never look like moss.
Will