Are you thinking it is green algae and not mustard? It seems to be in only the areas where it is shady most the day. It's not on the walls just on the slopes and in the rounded area where the wall meets the floor.
I always find it hard to determine the algae type from a picture. I think there's a smear test you can do where you try to rub some off with your finger and then onto a bright white card to see what color the algae is.
Either way it definitely looks like algae to me and I have read that algae can get into vinyl seams and be very well protected from chlorine exposure. I have no idea how to get at it other than sustain shock levels of chlorine and vigorous brushing.
Since your pool seems to be very reactive to algae growth and shaded a lot, you may need to consider secondary sanitation methods such as adding 50ppm borates to the water or using weekly maintenance doses of polyquat-60 algaecide. You should also consider keeping your target FC at a higher limit to ensure a larger FC/CYA ratio which is effectively a higher kill dose of hypochlorous acid.
Finally, if secondary sanitation methods don't yield cleaner water, you may need to test for algae nutrients in the form of nitrates and phosphates. Some folks have used phosphate removers to lower phosphates below 100ppb which then makes the water less favorable to algae growth should your FC accidentally drop below an appropriate kill level. Nitrates can only be removed via water replacement. These methods of nutrient removal only help if the FC/CYA ratio is consistently observed since that is what actually kills algae. Making the environment starved of nutrients only acts as insurance should the FC/CYA ratio accidentally drop below proper sanitation levels.
I also noticed in your signature that you don't mention your chlorination method? Do you manually dose with bleach everyday? Automation of sanitizer dosing can help a lot too. You might consider either a salt water chlorine generator or a Stenner pump for bleach dosing. In your situation, keeping your daily FC swings to a minimum would help a lot too.
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