Help with creep material, utilizing render target

Hello fellow unreal engieers!. Im a bit over my head and would appreciate some help…

Im trying to do a zerg like creep material. When an object move upon the ground, the ground will be affected and mask to a different material. Also slowly lerp back to the orignial material when the creep objects is no longer nearby.

What I currently have is a material that use scene capture that draws additive on a render target where the objects move. I have few problems that im having a hard time solving.

The first, and biggest problem is that I dont know how to lerp back to the original material on the places when the creep object is no longer nearby. If anyone could help me with this I would be very grateful!

Some minor problems that are not as big but would still appreciate the help.
My render target texture is now black and white, and the edges are very hard. I would like more smooth edges, blurry. Can’t find any good way to solve this either.

Thanks in advance & keep hammering!