its common because you are leaving your AC on recirc. the moisture inside the evaporator (iirc) is not being allowed to vent out. thus, when you get out of your vehicle, put it on vent/fresh and let the moisture vent out.
there are a number of aftermarket treatments for it. what they do is drill a little hole in the evap box and then spray a chemical in it to kill the mildew. i betcha your dealership has it for about $99. i forget the name of the company, but you can also buy/DIY