That’s the back side of a register in his own house, that was regularly wiped down and vacuumed from the front side. He’d taken it off to clean the inside of the vent, which was quite clean (he changes his air filter regularly!) and entirely mold-free. He hypothesizes that with the air-conditioner running, the inside gets colder than the outside, and condensation forms on the colder surface.
All the registers in his house have since been scrubbed squeaky clean, but we thought we might share the information….
Most registers are only held in place with two screws and come down easily with a screwdriver (usually phillips). A quick scrub in a bucket of bleach, rinse, dry, and re-install.