Mold in air conditioning is from standing water in the system. There could be a small leak in the drain and getting the cabinet just wet enough to grow a mold or the A-coil drain pan has the mold.
Check to see that the cabinet is dry. If not, clean it with bleach or a mold killer and repair the leak.
Otherwise check the A-coil drain pan. Think of the A-coil as a car radiator with a drain pan underneath it. As warm air passes through the cold A-coil, water condenses on it and drips down to the drain pan. The water collects in the pan and drains out through the PVC pipe on the side of the furnace. The drainpipe is usually about four to five feet high and runs to a floor drain or condensate removal pump that pumps the water to a distant location.
You could try removing the humidifier and looking in the hole in the plenum to survey the situation.
If there is no access to the drain pan without cutting and you do not want to cut the sheet metal to get access you could try cleaning the drain pan from the outside.
Usually you have to cut the PVC to unscrew the pipe or first fitting coming out of the side of the furnace. You may find the pipe partially clogged and this is why there is standing water-growing mold. Depending on the water you drain out after the first cut of the PVC could control the next step.
I would like to flush the drain pan with some household bleach and water, mix 1 part bleach to 6 parts water. Get an adapter that will screw to the outlet of the drain pan so you can attach a hose to it. Do not forget to get the parts to put the drain assembly you cut back together. Put a funnel in the hose, hold it up, and pour about 1 qt of bleach and water in it. Hold the hose up for 20 seconds and (with a bucket handy) pull out the funnel and drain the mixture out. Repeat this process several times until the water returns fairly clean and reconnect the old drain assemble, make sure it is not clogged.
Another possible solution to a stinky area is with ozone. I use it in my ventilation system every couple years during spring cleaning. I take an ozone machine and place it in front on my largest cold air return. I set the machine on high, turn the furnace fan control on, and wait until I can smell ozone come out of the vents. It usually takes 2 1/2 hours, but I have a small house.
The ozone machine can also be placed at the base of the furnace if there is a cold air return installed near the floor.
If you are nervous of the ozone, set up the procedure and leave the house for a while. When you return and shut off the ozone machine the ozone only has a life of 10 minutes or so.