Even though someone else hit your car, it is your "litter"--deris from your car--and you have to clean it up, the same way that if you set out your garbage or recycling for pick up and it is knocked over through no fault of yours (e.g the wind; local kids) you'd have to clean up the trash. If you don't clean it up, if anyone is injured by stepping on it or car damaged by driving over it, you could be sued by them for their injuries or costs, since you allowed a potentially dangerous condition to persist. Depending on local ordinances, you may also be subject to fines. You are liable for your own property and its clean up, including after an accident.If you incur monetary costs in cleaning up (e.g. have to hire someone or some service to help clean), you could include that cost in a claim against the driver who hit your vehicle.