[JLMounce]

Definitely looks like mold spores.

I would start by getting some ventilation in the car. roll the windows down even just slightly. You need to get it dried out in there.

I've never dealt with anything quite that bad, but you want to try and remove that mold/fungus without just spreading it around, as it'll come back. Hit your local car store and try an interior carpet cleaner. Those are a bit harsher as it's meant to deal with the crap on your feet etc. Work sections of the interior at a time and get this up as by doing it as dry as you can. Get some towels and scrub pads that you don't mind throwing away after. Instead of spraying the surface directly, spray the pad and whisk it out before applying to the interior. Spraying the material directly will just spread that stuff around.

You'll want to clean the carpets as well and you may need to take a peek under the carpet as you may have moisture buildup underneath that is contributing.

If you live in a humid climate, you can try a couple dehumidifiers made for guitar cases.