Apparently this wasn't a problem when copper was used in the cooling coils. Copper is an antimicrobial even in very small concentrations
Some have suggested/claimed placing a few small copper rods in the drain pan helps keep the biofilm from forming.
We used to use it to keep algae from forming in our pondless waterfall.
Another suggestion in this thread:
Cleaning slime mold from condensate drains
I found some large 1" crystals of copper sulfate and put one in each drain pan.
The copper sulfate crystal very gradually dissolves in the condensate and keeps the drain and the pan completely clear of mold and slime.
I was doing this for my customers back 50 years ago when I owned a HVAC business, and continue to do it for my own condensate pans today.
You will never see mold growing on a roof below copper roof flashing. The same is true there.
Copper is a mineral and it's found in small quantities in the earth everywhere, so non-polluting. Most houses have copper plumbing with drinking water running through it. In high quantity, like the crystal, it would be dangerous if you ate it though.