Contrary to popular belief, GFO doesn't kill anything. It's not an algaecide. It simply soaks up PO4 before algae can utilize it and grow. Sure after a while some algae will go grey & slowly fade away, so I guess i can say it can kill albeit slowly. That's why eradication first is the best way to go. I mention this bc when people say "it didn't work" I often wonder what the actual problem was. Because no plants can grow without nutrients but if deprived long enough, although they can hang on for a while but won't actively grow.
The reasons are many IMO. Changing it late, improper flow inside the reactor, poor distribution of output water in the tank, not enough media, overwhelming inputs, impatience etc. I helped someone with big B. Once I saw his system in operation in person, the problem was obvious. He thought GFO was to be changed every 4 weeks. And 4 weeks sometimes became 5, 6... But my test showed significant amounts when tested at 3 weeks. The reactor was also mounted not level, and the water was channeling.
So my point is I think some who experience a major algae problem of this type for the first time misunderstand the role of GFO and may make mistakes while implementing it.