How can I make 1 material shader reveal another?

What im trying to do is make my footstep decals reveal the grid below them as my character walks. All I have is a simple plane with a grid material thats emissive turned off. I want my foot steps to reveal the emissive grid as the character walks and then slowly disappear. Is this something that is possible?

i think to do that you would use either decals or a particle system. there may be a way to do it in materials but im not familiar with it if there is.