There are functions for everything in Bonsai. Mosses and lichens are used as ground covers.
Moss on Bonsai is usually just laid on to make the Bonsai look good for a photo shoot and removed afterward. Even traditionally, moss was usually used only in short term tray landscape plantings or applied to Bonsai soil just before a show.
Unless it is growing on a particular tree (e.g. English oak) that acquires it naturally by being outside in cool, misty locations, moss can be harmful to the tree. It is common to have all kinds of trouble with the moss because people end up watering the Bonsai too often to keep the moss alive, which will kill the tree. In certain cases Bonsai trees won’t get enough water because the moss is ‘intercepting’ it.
It is interesting to note that moss does not rot a tree (there are mosses that grow on trees naturally under cold and moist conditions). The problem with Bonsai trees is that the soil mix breaks down and this rots the roots of the tree. Often, a nice carpet of moss on the soil makes it hard to recognize the problem until it is too late.
Moss requires shade, high moisture, and a very low amount of nutrients. Just about the complete opposite of most trees’ requirements. But if you are very keen on having the soil of your Bonsai covered by moss there are alternatives to real moss. You could ask for Irish moss or Scotch moss in your nursery. These are not mosses but are vascular plants that are moss like in appearance and may be used as ground covers on the Bonsai quite effectively. However, they should be planted sparingly so that their effect is natural, and they do not prevent water from reaching the soil.
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