[MaSnaka]

Tim,
If I was your contractor I would be sick to my stomach. Normally just water should clean it off. Drywall and drywall mud are water soluble but can be abrasive. I would soak it with water to get it soft and gently wipe it away with a clean soft towel turning it to always have a soft clean edge touching the car's paint as opposed to smearing the residue as you wipe. Same with the sidepipe. Good luck.

John