I believe what you're talking about are the spores from Artillery Fungus.
In my experience, a softer clay works better than the harder clays.
Also, having a good coat of wax / sealer really helps. Washing the car weekly helps. Parking the car away from any mulch will help prevent the problem. Do NOT let the spores sit on your car for too long. This is fungus we're dealing with. It will grow on or into any surface it can get a hold on. My detailer has photos of a white truck (before and after) that had the spores sit on the paint for too long and the spores actually started growing down INTO the clear coat.