In bonsai often you will hear people talk of 'Sphagnum Moss' or also 'Peat Moss' or a combined name at times.
This can be confusing because there are more than one single product called this. They function in different ways as a soil amendment though.
One tye of peat moss you may have seen in bags at your gardencenter consists of very fine particles
almost like dust in fact.
This type is difficult to get wet and stays wet for a very long time.
Due to it's particle size being so small it also inhibits drainage and aeration.
This is the wrong
type for your bonsai.
Another kind is 'Spagnum Moss' or often called 'Orchid Moss' and found near the orchid supplies.
It looks like the picture here though you are more likely to see it in a package about half the size of a loaf of bread.
If it is stringy, Just chop it up with some scissors It need not be tiny pieces.
This is the right stuff
. It retains moisture yet will dry, It allows for air spaces in you soil [roots need air and water]=provides good aeration, and does not impede drainage. This is an excellent choice for an organic component to your soil.
See previous links for more info on soils and their composition.
It will help to lessen the risk of overwatering [Though that is up to you] And be doing your trees a favor