This will show how to make rocks which when placed randomly, will have moss on top.
Import your rock fbx files.
Create a new material and call it M_ mossy Rocks.
Create a material INSTANCE, and apply the instance to your rock fbx.
Create a Lerp (LinearInterpolate) Node, and pin this into the Base Colour slot.
In Slot 1 of the LERP, plug in a TextureSample Node, Containing a rock diffuse map.
In slot 2, plug in another TextureSample node, this time with the moss texture.
In the Alpha slot of the LERP, place a WorldAlignBlend Node
Add a ScalarParameter Node and Plug it into Blend Sharpness. Recommended value of 5.
Add another ScalarParameter into Blend Bias, and set it to -0.5.
The Lerp and textures can also be made for other settings such as roughness, Normal, etc, and the World align Blend can go into their respective alpha slots.
Not that instead of a moss texture, you can also use a Vector Parameter Node (Which is a colour node) set to white, to get snow instead.