Iron in water source

[BigDave]

... will have NO EFFECT on the algae, until your PO4 levels become lower than 0.25 ppm (250 ppb), and won't be fully effective till you have less than 0.1 ppm of PO4 present...

I was still getting mustard algae at 250ppb. It didn't stop until I got the pool under 125ppb.