Usually it is moisture and can't really be fixed since Air Conditioners condense moisture. Over time the dust and dirt with moisture creates mold and that is what you smell. But if you turn off the Air Conditioner several minutes before you turn off the car, the moisture can dry up preventing the mold to form and prevent the smell. I am not sure, but changing the cabin air filter might help by keeping the dust and dirt out of the air conditioning system.

My air conditioner started to smell, and when I turn off the air conditioner a few miles before turning off the car, the smell went away. If it is not hot, I don't even use the air conditioner. Now if I sometimes forget to turn it off, it still doesn't smell. As long as I don't let the dust dirt and moisture build up, the smell is probably under control.

I noticed that on hot days with long drives I turn off the air conditioner as I climb a steep hill, when I turn the air back on, steam comes out of the vents.

This is common with every car I have ever known usually when they are about 8 years old.