a filter in earlier automobile HVAC systems. Most of material occluding the evaporator are small fibers from the interior, and various and sundry atmospheric aerosols, aided by oil weeps from the refrigerant. It is also possible as you say, given the wet cool environment, that mold growth can be a contributing factor. If you look deep into the evap, the passages are free all deposits remain on the surface.
This is a good time to make sure the mechanism that controls the "blend" door/water valve are working correctly. The motor and/or its gear teeth can go bad will disastrous results for a/c function.
