Could be Cyanobacteria
Bel…
I saw your post yesterday but did not have time to respond. I have a 75g planted tank that has been up and running for about a year. It has gone through several different algae species as it has become balanced.
What you are describing sounds as if it may be a strain of cyanobacteria, which can be really nasty stuff. It will kill plants, smells awful, and defies conventional treatment.
I would recommend a search of BGA (blue green algae) or cyanobacteria and read about this stuff.
I would encourage you to never use an off the shelf algae killer as they will kill everything in your tank, maybe not the first dose, but it will happen regardless of what it says on the label.
To get rid of cyanobacteria, if that is what you have, you may need to resort to antibiotics, a controversial step. I have used them once with great success, but not without a lot of research and even a consultation with my physician ( gram – gram + bla bla bla). I decided on a procedure, duration, and strength etc. that seemed right for my tank and did it. I came thru it bio filter intact and cyano gone.
Good luck