Green dust algae forms a green film on rocks and most noticeably on walls of planted tanks. It is easily wiped off but comes back quickly. In tanks with strong light it can reappear within a day. In less severe cases, it means weekly scraping/wiping of tank walls. For aquascapers that only have had tanks that suffer from chronic GDA, wiping the glass on a weekly basis might seem like an inevitable thing - we assure you that it is not. GDA is differentiated from GSA (Green spot algae) easily as it grows much more quickly (with no particular dot formation) - and forms an obvious thin green film on tank walls.
GDA is more common in "hot" tanks - tanks that have tons of lighting and nitrogenous waste. Ammonia or nitrogenous waste can trigger it easily. Tanks with high NO3 levels (above 10ppm) and not enough plant mass are also very vulnerable to it. Many EI based tanks suffer from this. Cooler tanks, with more moderate lighting, leaner dosing regimes have much less issues with this.