So you brushed this fairly vigorously and it doesn't move at all? If that's the case, then it's unlikely to be either algae or pollen (only black algae would tend to stick to the floor or walls so tightly, especially in plaster) so perhaps this is metal staining.

Is this a plaster surface (as opposed to vinyl or fiberglass)?

If this is staining, then it could be an organic stain, say from tannin from leaves sitting on the bottom, or it could be metal staining. If this is a plaster surface and you take a Trichlor puck and put it over one of the stains, then if it's organic, it will lighten up and go away (do NOT do this if this is a vinyl surface -- the acidity of Trichlor is VERY bad for vinyl). If it doesn't, it's more likely to be a metal stain. If it's a metal stain, especially an iron stain, then putting ascorbic acid (say, a Vitamin C tablet) on the stain would have it fade.

So you have some things to try, but if your high chlorine isn't changing the situation, let the chlorine drop back down. Measure your FC level tonight and then tomorrow morning before the sun hits to see the rate of FC drop. If there is a lot of algae or even pollen, then the FC can drop overnight, but with stains it usually doesn't.

Your pH measurement won't be valid at high FC levels.