In my experience, mustard algae never really go away 100 %, since I do believe it comes from the rain. You'll notice that it pops up more after a good rain, than if there's a period of dry weather. If you're in the midwest, this year it's seems to be a bit more frequent. I've talked to other pool owners in the area and they're saying the same thing. Last year we were in a drought, so i didn't see much of it except for the few times it did rain. Just have to keep the Cl level a little higher this year. I have an AG 20,000 Gal vinyl and keep my Cl between 5 - 10 ppm, and shock at 15 - 20. Doesn't bother the liner at all. If your Cya is high, you may have to shock higher. Check the chart. The key is to vacuum it as soon as you see it. A third of my pool is in the shade from a backyard tree for about 8 of the 13 hours of sunlight I get in mid summer. I'll notice maybe 6- 8 little half dollar sized spots come up every 4 -5 days in the shade. No big deal. I just vacuum them up and they're gone. The algae isn't very hearty, it's more like a dust that settles at the bottom, as I thought this was the case at first, until I did research on it.