I had mustard algae and had a heck of a time getting rid of it. It wasn't a bloom, but it would grown on walls, steps, etc. and be back the next day or two. What finally worked for me was to add 1 qt polyquat 60, then get chlorine up to 20-25 ppm and hold it there for 2 days, run filter 24/7 and brush a couple times a day. After holding chlorine very high for 2 days I then let the chlorine drop down to 4 ppm, put in another quart of polyquat 60 and it's been fine ever since.

I think the main thing that helped was getting the chlorine up to 20+ for 2 days. Before I would just shock it and let it come down and while it did seem to kill the algae temporarily it always returned later. I have a pebble finish which is worse than standard plaster because this stuff gets into all the tiny nooks and crannies.

good luck.