You have some fungus or mold growing in your evaporator tray behind the dash. First make sure the drain hose is free of any obstructions. It usually comes out of the firewall in the middle.
Once you have confirmed this then spray some Lysol or other disinfectant spray into the air intake and have the fan on high. You may want to take the cabin filter out and spray into that with the fan on.
Unfortunately, I have been down this road a lot. You can blast the heat on high for 10 - 20 minutes, also engage the defroster with the heat enabled.
My favorite product is BG Frigi Fresh. This you spray from the outside intake vent. They are right below the windshield. You will see the plastic trim and crate style opening. Turn your AC on, get out of the vehicle, and you will hear where the outside air is being pulled into the car. With the AC on full blast, spray the product in 10-15 second bursts. I then alternate the AC recirculate a few times. A step further is removing the cabin filter and spraying the return vent on recirculate mode.
Now, with my Hyundai’s, I catch the mildew smell when I let my fan blow after disengaging the AC. This sends all the condensation into my vents and the problem existed there. I no longer practice this method on my Sonata nor Tucson. The AC is disengaged and the fan off a few seconds after. I have had no more problems. This is contrary to the standard protocol.