Sure, I’ll make the moss texture a parameter … make it sand, moss, whatever … or turn it off entirely. 
EDIT : BUT to explain it a little more techy … basically, Epic added a neat feature where the lighting rebuild results can talk to your material and tell you where the mesh is occluded. So, dark corners and such. You can then use that information, like a mask, to add effects like I’ve done here with the moss. It’s cool because it isn’t baked into the mesh or into the textures … it’s entirely dependent on how you place the meshes in your level.