This tool allows you to generate moss on the surface of the meshes, by simulating the natural build up of the moss  and creating individual strands of fibers to ensure maximum realism.

Everything is procedural and you can customize color, age, the length of the fibers, the size of the textures pattern, control the region were it grows with a mask and with filter based on the mesh normal.

It might take a few seconds depending of the settings and the amount of fiber instances, but when it finishes to calculate, it will render really fast, as it doesn't uses transparency. Also, as it uses instances, it will not require a lot of GPU memory to render.

It can also remove moss fibers that are not visible by the render camera, improving the performance even more.
You can add this to meshes that are inside a collection, so you don't need to do this individually.
To use that you just need to Append the "Moss.blend" file to your scene, selecting the "Moss (simple)" inside the Nodetree folder, load the Geometry Nodes modifier on your mesh and select the Moss(simple) or (advanced).

Watch this video to learn how to use it;