IF, a very big IF, you can clean the duct. (rotarary brush will damage the board as will and air whip) but a straight air nozzle with no whip will also remove MOST of the dreaded mold. Then it can be coated with any one of sevearal sealers and fungacides (like Fosters').
Your a/c either has wet cold air blowing through it or saturated wet from the wet a/c cool throughout the summer. This ideal for mold growth. To prevent mold growth in your a/c ducts, fiberglass or metal, dry thoroughly a couple hours every day. This requires <50%RH air inside your home. Operating your fan for two hour each day when the a/c is not operating is one way. Another method is to discharge the output from a whole house dehumidifier into the a/c supply duct. The units blow 200-300 cfm of dry air. Warm dry air will dry the a/c ducts dry enough during the "off cycle time" to avoid mold growth.