My 2009 Corolla has a problem with a musty, possibly mouldly, smell coming from the vents after I run the A/C for awhile. This is most noticeable when I run the A/C then switch to vent without A/C. The smell persists for at least an hour after I finish using the A/C and tends to still be there the next day as well. The car is under warranty and the dealer has tried things, including new cabin filter, checking the drain and spraying something into it, none of which made any difference. They also replaced the condenser/evaporator, which was a big job and solved the problem for about a week, then it came back.

Possibly an industrial strength deodorizer was added to the vents to give you a nice impression of the service. However, in this case it may have been overkill.

I had a 91 that smelled of gas because the vent line to the file hole rusted out. It got to where it would leak out on the ground if I opened the nozzle all the way up when filling.

this is a good possibility. if its green its moldy, does the car have a musty smell in it when it sets closed up for awhile? pcm is up under the dash behind the glove compartment. so any water that gets in will run over it. I would find the water issue if its the cause.

Does the smell happen if you are running the hvac on just vent ?
The ac itself would not cause a smell unless the clutch on the compressor were slipping and you would probably hear that.
Its a sealed system just like your fridge at home.
So it could be something in the engine compartment that is coming thru the vent when the ac is on. The most common smell is a musty or mold smell from moisture inside the hvac.

Check your valve cover gaskets, bet they are leaking and that is burning off.

I think you may need to chnge your cabin air filter if it smells like mildew or a musty type smell, its located under the hood by the windshield on those I believe.

Bacteria and mould can grow on th AC evapourater. There is special AC disinfectant stuff you can buy to run through the heater system to remove smell and nasty stuff.

the musty smell is from the dampness in your heater I would get it checked at a good Rad shop

It sounds to me if you are having a "musty smell" then you might have a problem with your condensation drain line for you a/c system or a small leak on your heater core. If it is plugged then water will collect in the vent system causing a musty smell from the water not draining and if there is a small leak on the heater core then coolant can collect in the vent system. I would run the a/c on a warm day and look for water draining from the vehicle around where the dash is located to check the drain line. As far as your air cabin filter most are located behind the glove box. You usually need to remove the glove box to get to the filter.

First thing I would do is rig a vacuum cleaner hose that will go deep into the defrost vents... cut the defrost on HIGH - WIDE OPEN - FAN ON HIGH!! -- then stick the hose down and dig around.. you will find you have collected a LOT of farm "dust" (down)(the way dust falls) in the defrost vents.. and the A/C vents are not (down) and nothing falls (down into) the vents...