Mustard algae or pool staining?

I live in Mesa,Arizona where the temperatures are now 100+ from now until probably October. I have a 25,000 maybe 30,000 gallon in ground plaster diving pool with a DE Filter. My pool has what I think is mustard algae. At first, I thought it was staining. It was a yellow-ish brown and started on the deep end of the pool. When I tried to brush it, nothing was happening. I bought a wire brush and stain remover from the pool store. Two days later it was on the shallow end and that is when I realized it must be mustard algae. I used about 14oz of copper algaecide, let the filter run overnight and then supershocked the pool with 7lbs of granulated shock with the filter left on. After 24 hours what I thought were stains had lightened significantly. Now the pool looks like it has freckles, but only from the pool slant down to the deep end. Here are the last results from the pool store:

Free available Cl 1.51
Total available Cl 4.73
Calcium hardness 908
Cyanuric acid 165
Total alkalinity 165
pH 8.0
Acid demand 4
Copper 0
Iron 0
Phosphates 500
Total dissolved solids 3200 (unable to drain the pool this time of year due to the high temps)

The pool store advised me to add 1/2 gallon of muriatic acid, run the pump for 4 hours, then add 3lbs of shock. They also had me add a phosphate remover over a few hours time which totaled 3 liters. My pool still has freckles......