JI'll keep experimenting around to see if there's a dip dose of Excel (glutaraldehyde) that I can do to kill this stuff. If not I'll move to vinegar or citric acid and possibly combinations of Excel, H2O2, acid...
Will be interested to see your results. I do know:
1) H2O2 becomes a much stronger oxidizer at low pH. I've used it in combination with vinegar for cleaning driftwood and such, but only outside the tank.
2) Algaefix (Busan 77 and equivalents) increases absorption of Excel (glutaraldehyde, Metricide, etc.) and therefore increases the biocidal potential. Metricide 28 is a combo of these two ingredients.
Stef also made a discovery recently. She added dried banana leaves from our yard to a small pseudo-planted tank, intended as a conditioner for bettas, but it also made some BGA that was present gradually disappear. Repeated it in another, properly planted larger tank with BGA on the filter outflow - same result. I'm not sure if it's working in the same way as things such as barley straw are speculated to work (gradual release of H2O2), or if it's something specific to the banana leaves. Also did a test with it against thread algae, but no effect.