I've had a lot of issues with mustard algae over the years. It has been all about the extremely high CYA in my pool. I was using the stupid trichlor pucks as my company suggested. I drained my pool last year, but over ran the CYA level (70). I drained again this spring and was extremely careful to get to only 30 ppm CYA.
This year, I've been running my FC up to the top of the recommended level every day. I sat down with the charts from the CYA and found a suggested higher HOCL.
The HOCL line for bacteria is about 0.011 ppm. For Algae, it is 0.050 ppm. For Mustard, the number recommended was 0.070. With 30 CYA, this is a minimum 4.3 FC. For 40 CYA, this is abut 5.7! This is one of the reasons I'm running only a 30 CYA instead of 40. And about 7.2 for 50 ppm CYA!
I lose about 2.5-3 ppm per day in my pool. I was running the FC up to 8 to keep it above 5. I have switched to raising it to 6 PPM FC each evening. Next morning, it is still 6 FC. It degrades to 3-3.5 FC during the day and then I raise it back to 6 PPM. I have not had a mustard outbreak since I switched to this. I think part of the reason is the pool stays above the mustard killing level for at least 18 hours per day, if not more.
I couple more things, I am running my VS pump at low speed 24 hours per day (about 20 hours at 1050 RPM/40 GPM). I am also now running about 27 PPM Borates, which will be raised as soon as my boric acid arrives. Using CalHypo 73% for chlorine, to also raise my CH level (320 ppm). As soon as I reach 350, I'll switch to liquid bleach 10%/12%.