There could be a number of causes of this residue. Sometimes it is due to protectants used on the dashboard to make it shiny and keep it from cracking. When the vehicle is parked in the sun, some of the protectant evaporates onto the inside of the windscreen. Using a piece of scrunched newspaper and window cleaner can be an effective cleaning method. Alternatively, the residue may also be caused by a faulty air-conditioner unit leaking gas. The gas looks like frost or icicles around the vent areas. If this occurs the window may need to be restored or replaced, depending on the amount of damage, and of course, the air conditioner will need attention, too.