Odd Stains, not Algae

[GJBenn85]

For a few months now I have been fighting a losing battle with stains in the shady side of the pool. The stains simply do not brush off and they appear to actually be in the plaster; no slimy feeling, nothing brushes off, so I concluded it is not algae (have also been keeping the chlorine higher and been using polyquat). Staining is yellowish-brown in color.

The worst area is a bench on the shady end, second worst being the walls surrounding the bench:




In the first photo, the lighter area about half way over at the edge of the bench is what the aggregate finish should look like. The second photo better shows what I am dealing with.

Pool numbers:

pH: 7.4
chlorine: 4
TA: 80
CH: 200
CYA: 40
salt: 2800

Any ideas on what this is and how I can get rid of the staining?