I almost can't believe I'm saying this, but with 0.5 ppm phosphates, it might be worthwhile to try using a remover. I'd recommend wait for the new filter, though. The removers cloud the pool badly, and it will clean up better with the larger filter.

Meanwhile, keep the chlorine high.

If you want to do so, adding borax to reach a 60 ppm level would also help. On a 23K gal pool, you'd need about 100 lbs (22 boxes of 20 Mule Team borax) plus about 6 gallons of muriatic acid. Borates at that level do seem to inhibit mustard algae, from the reports we've seen.

PAC floc will also remove phosphates, if you use them as a drip feed ahead of your filter, and clean the filter before the floc can escape. But you'll DEFINITELY want the new filter first.