Well, I thought I had done things right last time when I attempted to get rid of the mustard, but evidently I didn't. I'm pretty darn sure the stuff is mustard, as it started in the mouth of the skimmer, and then appeared in pool/spa light areas and on the sides of the pool where it's shady.
When trying to get rid it a few months ago, these were the numbers:
pH: brought it down to 7.2 before shocking with liquid chlorine (normally 7.4 - 7.8)
CYA: < 30 (30 is the top mark on my test, and I could still see the black dot)
I brought the FC up to 30 ppm to begin with, and kept it between 25-30 ppm for about 4 days. I brushed a LOT during this time as well. After that I let it drop to around 20 ppm for another day or two. Then I verified that it was holding overnight so I stopped adding chlorine (it may have been holding overnight earlier, but I hadn't checked yet). I tried to make sure that I kept the FC at least 4 ppm for the next several weeks.
Things looked GREAT. For a couple of months I didn't see a hint of the stuff anywhere. I was HAPPY! But then I started noticing it in the skimmer again, and I knew it was back. There may have been a day here and there that the FC dropped to 2 ppm or so, but for the most part I've kept it higher. I get the feeling that I didn't get a new infestation, but that I didn't really get rid of it all the last time I tried. But of course I could be wrong.
I'm wondering if I've overlooked some things in getting rid of the algae. How often should I be backwashing my DE filter when doing this? Could I be re-introducing it with things like the hose, brush, pole, net, or vacuum head? This time around I may also remove the main pool light, as I may not have been able to brush some algae off from under the metal ring. Anything else? Should I do the same thing but for a longer period of time? I'm due for a complete DE filter cleaning (I like to take it apart and clean the grids each 6 months). Should I do that before starting the next attempt, or should I shock and brush for a while, then completely take apart the filter and clean it?
My numbers are still the same for TA, CYA, and CH. I wasn't going to bring the CYA up for the Summer until I had finished attacking the mustard again. My pH varies between 7.4 and 7.8 (it's always wanting to rise and I bring it down a couple times a week). I'll bring it down closer to 7.2 before shocking. Currently my FC is 4-5 ppm. That level is keeping the mustard from getting much worse before I attack it again.
Any suggestions would be greatly appreciated.