You're sure its green slime algae?
Its known to be caused by low nitrates, also very common to have it on gravel and up against the front windows (where it gets more light). If your tank recently cycled with high ammonia, that made it "kick off" so to say.
Dirty Gravel (not vacuumed) and / or dirty filter can cause this too. So vacuum well when doing w/c and clean your filter (in used tank water only to not kill off the beneficial bacteria). Also make sure your water outlet is set in a way to circulate all areas well (it often grows in "dead" corners where it gets no/ little circulation).
What worked WONDERS on mine: 3-4 days complete black out, tank fully covered with heavy dark towels, no peaking no looking no feeding fish (yes the fish will be fine to not eat for few days). Then I super vacuumed the gravel adjusted my outlets and its been gone since then.
Also check few times during the day when the sun moves around your house if at some time this tank is hit for a while by direct sun light, if so either cover the tank during that time in that area or let down the blinds etc.
I also read articles in which it was suggested to block the front of the tank maybe like 5" upwards with dark tape so the gravel doesn't get the "outside" (room) light. Personally I never tried this, since I solved it with blackout, filter cleaning & adjustment.
IMO chemical "attacks" on a tank should always be the very last resort if all else failed. Chemicals in tanks often work two way either they mask the problem, which doesn't help any in the long run or they eliminate the problem indeed but at the same time also eliminate all beneficial bacteria - Either of the 2 cases will then create a whole new set of issues & possible death in your tank. Just so you understand why I said "I wouldn't do that" :)
Sorry its became such a long post, just wanted to give you all info I know