The only way I can think of to remove ground cover is to paint under the decal with another terrain material.
That is how I do it, usually some dirt texture, which adds additional detail and realism to the level, since roads are not build on grass, in reality they clear the area before they build roads on it. If you really want to keep it grass you can make a second terrain material with the same texture, but remove the ground cover from it, so you can control where the ground cover is.
Ohterwise I would advise not to use my assets from my old packs, but use them from the actual Uebergame repo, since they are often the improved versions and bad ones sorted out. The grass you are using is one of my first attempts, which I was not satisfied with.