Paint one material on another ?

Hello, is there a easy way to paint something onto another ? Like a material on another one, or even a texture on a materiel ?
I know about vertex painting but i can’t make it work with a material.

I have a house for example, who have a material applied on it, i just wanat to paint some moss on the roof. I can’t find any way to do this… some will say to apply vertex painting on the roof but it’s the same, i’ve seen plenty of tutorials on how to vertex paint, but my roof material i way more complex than just 2 textures and 2 normals like every tutorials show you.

I bet it’s easy to do, but how ? Thanks !