golden brown stains on vinyl liner

[klasikal88keys]

Hi. I'm new here. I've been looking for some solutions to our staining problems. We have a 24 foot 13,000 gallon above ground pool that has golden brown stains that look like dirt piles on the bottom of the liner. These 'piles' don't vacuum or brush up. The pool store said it must be mustard algae. We've treated it 3 times so far, and the stains are still there. I re-adjusted the Ph recently because we had 15" of rain in this area. I'm currently treating it again and thought I'd post some pictures in hopes that someone might be able to help.
Thanks in advance.