Thanks dom,

I always thought that green dust algae (GDA) is quite different from green spot algae (GSA). GDA something like dust or thin film usually on the glass. With GDA, we can easily wipe if off. But GSA is hard, circular and bright green spots, difficult to wipe it off unless scraping with a razor blade.

Interesting suggestion from Tom Barr to cure GDA as I quote his statement in this link : -

"The cure is deceptively simple:
Leave it the heck alone.
Do nothing, let it grow, do not wipe the glass, after about
10-20 days, the film will produce patchy thicker patterns and
parts of the film will fall off. At this point you can go in and
clean it off. I've done this in 9 tanks now(100% success rate) and several
other folks have reported success and no return of the alga
after a long bout.- Tom Barr"

Explanation why this works is GDA is zoospore, very tough, can reproduce asexually, once GDA goes through it's 'spore' stage, it is not an issue anymore.

In short, leave the GDA alone, typically 10-18 days, then wipe it off.

Also interesting to read his somewhat simplified statement on algae issue :-

"You'll note over the years I've shown that: BGA=> NO3
related(easy to cure with blackouts/NO3 dosing), BBA=> CO2
related, GW=> NH4 related, Staghorn=> urea/GW related, Green
spot algae=> CO2/low PO4 related, GDA=> life history stage
related.- Tom Barr"

I am going to try his suggestion on GDA cure.

Best Regards