I just hope whatever is growing is not also growing on the inside of the door and it's just at the edge pf the window seal. I say growing because that's what it looks like to me.. mold growing..
If it is salt where would it be coming from?
Pushed up from the ground into the door out the seal? Seems soo unlikely that would be happening from looking at the bottom of the door.
Why is it only targeting that small area?