Planted Tank Guru
Join Date: Jul 2010
Location: British Columbia, Canada
Buy, beg, or borrow a UV sterilizer, if you don't have one, and run it 24/7 for about a week.
Once or twice each day, brush down the green dust algae off the glass, plants, and wherever it sits, to get it moving up & through the water column, to allow the UV to pick it up & destroy it.
Can't guarantee it will entirely get rid of it, but it sure worked for me.
GDA is caused by some imbalance in the tank, unknown to me, and it may return if you do eradicate it, but it didn't in my case, and has not returned since.
Best of luck to you.
P.S. I tried a number of things first, including not disturbing it for 3 weeks, as is given as a common remedy, but nothing worked, till I used the UV.