Moss on rocks is made by adaptive subsurf and microdisplacement in Blender2.78. It is fake hair by tiny extrusion from surface.
The merit is the very low file size. Also, with adaptive subsurf, many objects can be rendered in relatively small rendering time. But if the camera is too close to the object, rendering time will be long.
Original post: Blender: Moss (Fake Hair) by Microdisplacement.