At least take off the vent for a picture. It Could be mold. Mold is EVERYWHERE in our lives. It lives quite well in our HVAC systems and attics and baths. Where ever people find comfortable temperatures, and not too dry air, mold lives too.

It is mold or bacteria. There are plenty of labs that test. You could get a swab and send it in, but why bother, since neither is good. You need a moist environment and nutrients ("dirt") for microbes to grow. Once established, you will have to be persistent to keep it from spreading. Clean it out with a combo HVAC cleaner and disinfectant. Mix it up, if it requires dilution, and put it in a small tank sprayer or spray bottle, anything that you can use to hit the insides of the box in question. I like a couple of NuCalgon products, Evap Pow'r C and Evap-Fresh. Even if a product says no rinse required, I rinse, but leave enough dwell time for the disinfectant/cleaner to work. I use foaming cleaners, though expensive, when coils are clogged. Coil cleaners should be slightly alkaline, not acidic, to avoid metal damage. Any quaternary amine disinfectant should work well as a high pH yet mild disinfectant with detergent qualities.

I can't tell where the mold is growing in your pic, but keeping the whole thing growth free is important. Stuff will start to grow on wet coils and where the water drains right after you get done cleaning it. You will immediately start to form a biofilm. I'd use some "pan tabs" or strips and place them on top of the coil if you can fit them. That will result in continuous suppression of the microbes. It used to be that coils contained copper and the constant dissolution of the metal suppressed microbes, but aluminum can't do that so we have to resort to chemical disinfectants. Some zinc strips might work on the coils to kill microbes, but copper is more noble than aluminum so it will cause the latter to dissolve. I think zinc is shown to work for bacteria, but I don't know about fungi.

Sophie Wheeler is right. Microbes are everywhere and waiting to colonize your wet cooling equipment. You never get rid of it so keep it clean. When you are done using it for any significant time, leave the fan running to dry it out. Use a timer to turn it off if you want to save electricity and save the blower motor. I'd use a plug in timer with the start pin(s) removed for a portable, but timers without captive pins are hard to get retail anymore. You can order them from Honeywell.