I've been managing my pool for a few years now, and have always been struggling with mustard algae. I can scrub and shock to 50+ppm, and it will all go away, until it rains or I have to refill the pool with tap water. I live in a tropical climate, and the fill water is well sourced. I know for a fact that there is algae coming in from the fill water, as I have to clean the foot valve on the well every year or two as it gets overgrown with algae 40' down. Yes, the well gets treated with chlorine, but it is a constant battle. Last week I stopped by the pool store for some more trichlor granuals to shock the pool and get rid of the mustard algae again, and on a whim picked up a bottle of copper algaecide. The directions called for about 7oz for the size of my pool, so I went ahead and put it in, along with the trichlor to shock it. Due to the amount of mustard algae, as soon as I scrub the walls (it had been about a week) the visibility drops to almost nothing. Because of this I wasn't able to scrub every square inch. Typically I will shock/scrub, let it mostly filter out, backwash, and then scrub again. After the pump had run overnight and the water mostly cleared up, I noticed something very strange. There were no "missed" patches of algae left! Presumably the copper had taken care of what was on the walls.
Since treating last week we've had two parties, with probably 50 people (including leaky kids!) in and out over the past few days. I tested this afternoon, and found chlorine levels at 16ppm, and CC at <0.5ppm. Typically after having that many people in the pool, especially kids, the water will be cloudy and hazy, however after an abnormally large number of people, the water is still crystal clear. In a few days it will prove whether the copper is doing its job or not.
Full test results:
Ph at 7.2
CYA at 70-80
Water Temp 80F
Salt at 3000ppm
Some borates in water, no test kit.
Phosphates at 250ppm, but that is a whole other can of worms
Also, it should be noted that nobody got out of the pool with green hair!