Stubborn waterline ring

I acquired a brown waterline ring over the winter that will not go away. I scrubbed it away with a pad and a diluted bleach agent and it has come back within 2 weeks with little pool use. I might add that I got some stains from tree flowers that ascorbic acid removes ( so ordered some bulk). So I am looking for suggestions to remove the waterline stain.:confused:

Fiberglass pool
7000 gallons
FC 5
PH 7.2
TA 150 (am lowering)
CYA 50
Phosphates 500
CH 120

I am using the BBB method and have been for over 1 year. The water is crystal clear no odor

I also add a capful of Pool perfect and phos free each week.

Thanks for the help

I usually use baking soda paste and a kitchen scrubber sponge on mine, others have suggested using Magic Erasers....

Janet