It is UV damage.

Armorall Trim and Plastic Restorer works really well when used liberally.

When it stated happening on my Jeep I used it about every two months for about a year. Each time I used about a half a bottle to do the whole Jeep. Haven't used it in about 2 years now, and my fenders still look new.

 

Looks like wax. That close to the painted surface it's most likely wax the dealer lot prep folks used. Try a soft brus with some simple green. Then use some 303 uv protectant.

 

Certainly looks like wax. The armor all product is nice but it wears off with water. I've tried every variety of "black plastic restoring" and they all suck. I do turtle wax ice (liquid) on the flares. It functions as a wax so it repels water instead of leaving water streaks down the flares. All of the protection without the chalky white you're seeing there.