Daljit Singh shared a mini-tutorial on how to add moss to 3D models using 3ds Max, Quixel Suite, Substance Painter, and UE4. 

Daljit Singh's tutorial covers moss cards, Fuzz map, and LODs setup. As a starting point, the artist used Quixel Megascans and Quixel's video tutorial that shows how to use Itoo’s Forest Pack plugin for 3ds Max.

Below is the final version. You can read the full tutorial on Daljit Singh's ArtStation.