Brownish-gray stains in vinyl IG pool

[gumbeaux71]

Hi all,
Has anyone dealt with brownish-gray staining in their vinyl-lined pools? (See attached photo.)

Opened the pool and had a minor mustard algae problem and lots of staining at the bottom of the pool.

I quickly resolved the algae problem with one dose of Yellow Out, but my first shock attempt did not resolve the staining problem.

Pool is vinyl, IG and has no heater. Put some Ph Down in a sock and rubbed onto the stain, which seemed to get some of it up, though very gradually and not enough to actually see the color of the liner underneath. I lowered PH to 6.8 and let chlorine go to 0 in anticipation of using some ascorbic acid on the stains. However, I just tested the ascorbic acid and that didn't seem to remove the staining.

I'm not sure where to turn and what to try next! HELP!

Thanks,
Erica