Then EM will not work.
I had several folks try EM on GDA at Sewingsalot' report that it worked.
It did nothing for several aquariums with it when those folks tried it.
GDA is an oddity.
Lowering light, so called brown outs does effect it.
I tend to not like those and try other approaches 1st. Blackout for 3 days only works on BGA, Green algae might grow slower, but when you increase light back up, just like the plants, they take off again. Algaefix seems to have killed some folk's cases, but others report it had little effect.
If you lack a real dense amount of plant biomass and have a lot of light, this also seems to be asking for GDA ISSUES. Some folks go months with it. I suppose I've been very lucky as all the cases I had lasted only a 3-4 weeks.
But it made testing it problematic, and inoculation seems impossible in a well run tank near as I can tell.
If it's GDA, then you'll need a different approach for sure, BGA, easy to get rid, folks have spent months with GDA.
H2O2 etc will do nothing to GDA.