It appears to be lint on the felt/rubber seal at the bottom of a window. If so put the window down, take some duct tape (I find it to be sticker than masking tape), spread the felt a little to get all exposed and use the duct tape to remove. Basically like using tape to remove lint from clothing. I have this problem when using a Chenille mit on the seal around the sunroof of my wife's BMW. I let everything dry well then tilt the roof open and use duct tape to remove the white fuzz.