Reveal material layer with decals?


I’m trying to create an effect for my game where you have two materials - a flat grey material and a regular diffuse material (brick in this example) and what I’d like to be able to do is have a decal that reveals the layer underneath.
Like you’re washing off the grey to reveal the brick underneath. Think Super Mario Sunshine.

This is my current material setup:

Problem is I don’t know how to go about setting up the decal to do what I need it. If someone could point me in the right direction it would be greatly appreciated.

Just hoping that someone could point me to the right section of the documentation so I can figure this out. I had someone suggest that it might be deferred decals?

No one has any idea what section of the documentation to look in for this sort of thing?