I let several black garbage bags (full of grass, twigs, leaves, and stuff) sit on my flat cement garage floor for perhaps several months. When I arrived to check on it, and move the bags to the curb, I found black gooey black stain-like stuff on the garage floor under the bags. I imagine that chlorophyll (decayed plant matter) and corroded plastic bad matter coated the garage but I am just guessing.
I did manage to take the bags to the curb, but I don’t know how best to remove what it left. What do I do about the stain? I have not touched it yet. I imagine that gasoline or kerosene and a scrub brush might work. Do you have any ideas?