That is "Rail Dust." I had the EXACT same thing on my White Platinum F150. I called the dealer and it is covered under warranty. I took it in and they took care of it. I got rid of the truck since then so I am not sure if they came back or not. They did an "acid wash," or so they called it, and then did a wax and clay bar.
I would call the dealer and make them take care of it, at least once.