Photosource it from nature and layer it on with GIMP. LDA_Bark01.tga in Kashyyyk textures. etc I might be able to get some good moss photos, it's how I do everything, if that is of interest to you. You can reskin the bark textures to have a repeating moss pattern. I might be able to take some of this work on for you, if you're interested. I think this would help you implement further geometry, blending the textures there.