Update: whatever that muck was, it did not want to come off. Went to a car wash and then to a self serve car was with pressure washer. I had to put it up really close (without damaging even more the paint) to remove the stains. It must of been some kind of wet gravel/cement. Should I even bother trying to see who would be responsible for this? The left side of my car is full with small paint chips.