Mustard algae is VERY resistant to chlorine . . . and most other things you might want to put in your pool. Using borax to raise borate levels to 60 ppm seems to make pools significantly more resistant to algae, including mustard algae. A 60 ppm borate level will NOT get rid of mustard algae, but it will make it easier to eradicate and less likely to return. It's a significant expense, but you only lose it from splash out and backwashing.

If you decide to do it, you'll need about 25 boxes of borax + 8 gallons of 31% muriatic acid. Add 3 boxes of borax, followed by a gallon of muriatic acid. Add the borax slowly, through the skimmer. Add the muriatic acid directly to the pool BUT read the muriatic acid guide, linked in my signature, FIRST. Wait 4 hours, then check your pH. Add either borax or acid to bring pH back into range. Then repeat, till all the borax is gone, and the pH is in a normal range.