It is so common that if you go to your friend's house (in Florida), you may see a version of it on their AC, sometimes darker, sometimes lighters, sometimes more, sometimes less, It is also common that many Florida Residents not even notice these marks on the AC until many years later where if becomes more noticeable.

Cleaning it is very simple and easy, it is not a complicated process. You may request services for a clean out and typically in a hour or two, those marks are gone, however they may reappear in the future since the typical Florida humidity and the cold-normal running conditions of the AC collided on different places of the AHU creating condensation, and that condensation over period of time may create those black spots again.