I have dumped powdered chemicals into the pool in the past. I honestly cannot remember if the "stain" appeared before or after I did that, but I believe it was after. I dumped powdered shock, yellow-out, some calcium chloride, and some stabilizer into the water (broadcasted it) several weeks ago. This was before discovering poolsolutions.com. I no longer do that, but this may have been why I got the stain. Not sure. I'll try to take a picture of it later when the stain shows up better (it shows up better in the evening, when the sun is low).

My chemistry is somewhat balanced:

FC: 4 CC: 0
pH: 7.6
CYA: 50
Total Alk: 80 ppm
Calcium Hardness: 500 (thanks to the crappy drops-based kit I got, see http://www.poolforum.com/pf2/showthread.php?t=1112 for details).
TDS: ? (never tested this, I thought this was one of those "pool store" measurements designed to get you to buy chemicals but has no real effect on pools)

Maybe my CH has been high all along, allowing junk to precipitate out? The stain looks very much like someone took a handful of a dry power and dropped it, since the pattern on the pool looks like that. I just wonder if that was the case, why it can't be brushed off with brush. How does it cling, and not get dissolved?