From The Aquarium Wiki
GDA stands for Green Dust Algae.
Green Dust Algae (Akistrodesmus), not to be confused with Green Spot Algae (GSA) forms a layer of green slime on glass surfaces. The GDA are free swimming, asexual spores and highly mobile. When wiping the glass the spores are released into the water, and will reattach to the clean glass surface to begin it's lifecycle again.
Let the green slime grow on the aquarium glass and leave it untouched for a period of 3–4 weeks to complete its lifecycle. After this period, lower the tank water level and wipe the GDA off the glass. The algae will not come back if done correctly. Any wiping of the glass before the completion of the lifecycle will release free swimming spores into the water and the process will have to start over again.