Vinyl liner stain, above waterline

[ckelly]

New to this, and to pools, but been lurking for about a year here. Anyway, I have a 2 year old 24' AG, 52", pool that has developed a dark brown "scum" ring just above the water line. It's almost sticky in some spots, and can be scrapped off with a fingernail to some degree. No pool store chemicals seem to touch it. It is mostly only on the sunny side of the pool (NE), the other side is nearly perfect.

The ring was lower earlier on with a lower water level, raising the water level made the ring disappear within a few days, now the ring is higher than the water level can be raised so I can't cheat and raise the level. The ring was appearing last year as well, and was more evident once it was drained for the winter. It's much darker and more widespread this year.

The pool didn't test for metals last fall, but I do have some blonde (green) girls that inhabit it most of the summer. I've been very careful not to buy anything with copper in it. The PH was low (6.8 or so), and the chlorine was low (.5-1) for a month or so this year, I won't get into the embarassing details. CYA is 42 by the way, and the pool is and always has been shocked every 2 weeks, if that matters.

I have a 24" sand filter, Hayward heater, Rainbow 120 feeder with 3" trichlor pucks. I use a fair amount of PH up to counteract the low PH of the pucks. Recently had "disappearing chlorine" due to an algae problem that was cleared up with the correct chlorine level, heavy lithium shock, and a bottle of 60% polyquat.
Water is now balanced, clear, and no signs of any other problems.

Any ideas on what this is, how to treat, and how to prevent?

Thank you for your help,
Clay