JasonLion,
I'm pretty sure it's the mustard. I got this last year and it appeared in the skimmer mouth and on the walls on the shady side of the pool. I tried getting rid of it using chlorine alone, and I wasn't successful. But I may not have held the levels high enough for a long enough period of time. By January, the yellow had started to get a little out of control. Brushing wasn't completely getting it off in some spots anymore. (I was afraid I had let it go long enough that I had staining). So this time I used a product called "Yellow Treat". (This particular product is 88.8% Sodium Bromide and does not contain ammonia). I used a small 5 oz bag (my pool and spa combined are about 12,000 gallons I believe). With this and a chlorine shock and hold, I think I got it. After a few days, all the yellow was gone, and I didn't see any for over two months. I just noticed it again for the first time yesterday. Because of the time between, I'm thinking this is a new attack, but I guess I can't be positive. I do know that I was under the wrong impression about the required FC level to keep algae away. According the chart (I have a low CYA level right now, about 30), the bare minimum FC would be 2.2 ppm. I was keeping mine between 1.5 and 2. I guess I know better now!
Anyway, what I noticed yesterday looked very familiar, and in the same couple of spots as it started last year. So I think it's the mustard again. I guess I can shock at a level of 18.2 or more, and hold it there for several days. But after that, what should I do? There's the "yellow/mstrd min" column which in my case shows a level of 4.6 ppm. I take it that after the several days of 18.2 I need to hold it at 4.6 for a while to prevent it from coming right back? If so, how long should I hold it at that level before I can return to the bare minimum/target levels (2.2 - 3.5 in this case)?
Thanks,
Greg