Remove loose mold by brushing the couch with a broom. You may want to do this outdoors. Wear a protective face mask so that you don't breathe in mold spores.
Use a HEPA-certified vacuum to clean the affected area on your couch and suck up the mold spores. Discard the vacuum bag so that mold doesn't grow there later.
Mix water with some detergent. Add color-safe bleach if the detergent you are using does not contain ammonia.
Sponge the moldy area lightly with the water and detergent mixture.
Wipe the furniture with a damp cloth that has been dipped in dilute alcohol (one cup denatured alcohol to one cup of water) if the mold remains. You can also try a chlorine bleach mixture (¼ teaspoon bleach to one cup water). If your couch is any color but white, use a color-safe bleach; even then, test it first on a hidden area of the couch to see if it affects the color of the couch fabric.
Dry the couch. Place it in a highly ventilated area where it can dry quickly, within the first 24 hours. This will prevent any mold from growing back.