Two miracle drugs, Chlorine and Persistence, may very well get that stain off.
I doubt anything will remove it immediately, but trying a few things and the continued exposure to the Cl in your water will probably minimize it to an aceptable level...maybe even make it go away.
I have seen several products recommended here on the forum but I have not tried any of them. Others will be along who have.
PS - Nice work on your water...numbers look good.
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