Yellow/Brown dusty deposits that reappear repeatedly

[matt4x4]

Hi Richard,
My CYA is steady at 20-30 ppm, probably started at around 30 in spring and slowly dropped over the course of the season to 20 as rain dilluted my water, I don't use much dichlor or trichlor, only a few pucks in the stairs to keep the chlorine concentration up inside them to avoid algae breakouts in there - very effective.
I didn't add any CYA this year because I opened to it being fine, and this late in the season I won't be bothered, rather keep the solar cover on during the day.
My alk and pH have also been in check all summer, it's been a relatively trouble free season, but every onset of fall I've seen this stuff creeping in, the funny thing about it is it is VERY slow to set in, you can keep your pool at 15PPM and it just slowly increases in mass, say over the course of a week, but you will not get it to go away, even if you don't do anything about it, it only creeps up on you slowly. The first time I saw it, I actually thought it was clay dust and attributed it to fields being tilled up the road, now I'm leaning more towards algae, especially since teh pool turns that tell tale blue once you do kill it off. I know ragweed is in bloom right now, I wonder if this stuff feeds off the pollen or something.
It's definitely not your typical common algae.
Oh, and tehy're not spots, rather like first described, it starts as a fine dust deposit in craters and at folds where the current would drop it, it eventually dusts teh whole floor if left alone, however, it does not "grab" on to the liner, even just creating a slight current above it will disperse it like dust and leave a spotless liner behind.

I just found this post - dealing with the same stuff - 20+ PPM cured it too.....although he's keeping the cl relatively high and still losing it daily - telling me the stuff's still lurking.....
http://www.poolforum.com/pf2/showthread.php?t=5471

And another - all in mid august onwards!

http://www.poolforum.com/pf2/showthread.php?t=5423