MOLD. It is not a problem with your vehicle. Any vehicle with AC is
succeptable to this problem. Caused by moisture condensing then a long
period of  non use of the AC letting the mold grow. I live in a cold
climate. During the winter my vehicles that are garage kept have this
problem, the ones that stay outside in the -0 temps don't. You can buy a
very inexpensive kit that will kill the mold. The one I use you simply drill
a small hole in the heater/AC box and squirt the aerosol liquid in. The
liquid then foams, completely filling your box and duct work with a foam
that will kill the mold then go back to liquid and drain out. Problem
solved. I simply put a small stainless screw into the hole so I can remove
it next year for another treatment. Or if you have concerns any dealer or AC
service shop can fix it pretty cheap.