The Focus I have at this moment is my third white car. I had this ''Oh Sh**'' moment too when I saw some rust on the side of my first white car back in 1996. After the initial shock moment...and a good wash...all of it went away.
It was and is still, brake pad metal dust that collects on the side of the car. Nothing to worry about. But I admit, it is freaky the first time you see it.