I'll help you get some phosphate remover. I'm currently using it, and it seems to work well, BUT . . .

You have to understand it's something an unrelenting process, that requires you to CONTINUOUSLY maintain a low PO4 level (< 0.125 ppm). Typically this will mean adding a small amount of PO4 remover every time you add water. It may require that you add a bit of clarifier to clear the resulting cloudiness.

You MUST understand that adding PO4 remover does NOTHING useful until you reach that 0.125 ppm threshold. If you reduce your PO4 level from 3 ppm (3,000 ppb) to 500 ppb . . . it may not help at all!

I'm trying to work this out on a 200,000 gal country club pool now. It loses about 1" of water per day, and is refilled with city water with 2 - 4 ppm PO4 levels (2,000 - 4,000 ppb), so it's necessary to add a small amount of PO4 remover at least every other day.

You'll also have to look at what chemicals you are using, and make sure you are not using anything that adds phosphates. If you have a problem with metal stains, this can be a challenge, since the effective metal chelants are based on phosphonates, and eventually release 'ortho-phosphates' (the algae-feeding kind). However, quite a few pool chemicals contain 'stealth phosphates', such as almost any product made by United Chemical (unless they've recently reformulated).

This REQUIRES that you use an accurate phosphate kit.

If you're up for this, I'd be happy to help. From what I can tell, it's pretty easy and not very expensive . . . once you work out a maintenance program for YOUR pool. In fact, I hope you do pursue this, since I'm looking for others who can test the process.

But, I'd be unfair to you if I didn't suggest a possible simpler alternative: draining and refilling! I do NOT know why some pools are prone to mustard algae. And in my local experience, draining and refilling does NOT solve the problem. But, it might for you, and then you wouldn't have to mess with one more factor (phosphate levels).

Lemme know what you want to do. Because of the kit cost, getting started with PO4 removal will cost you about $80. However, with a 5k gallon pool, you should be set not only for this year, but next season as well.