golden brown stains on vinyl liner

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.

Try putting a vitamin C tablet on the stain, if the stain lightens up or disappears, then it is a metal stain. If you rub some chlorine on it and it disapperas, then it is organic.

Today I did the Ph Decrease in-a-sock and the Vitamin C tablet check. It seemed like the sock didn't take much of the stain spot away, but the Vit. C melted a hole into the middle of it. Like I've said before, we treated for mustard algae 3 times and nothing has changed. What do I do if this is iron? My readings at present are:
Total Hardness = 400 ppm
Free Chlorine = 0/0
Ph = @7.8
Total Alk. = @160
Cyanuric Acid = 30-50
This is from an HTH test strip. We were told, too, that our chlorine would always read low because we have a Frog System on our pump.
Thanks

Levels from test strips are not very good, as they have too far a range to really know what is going on with the water. If you have a frog system, I think that uses copper - which can cause staining. With the levels you have, my suggestions would be to get your water tested by someone can test all your numbers without the strips. You can invest in a good test kit here on the Pool solutions web site, or you can get a decent kit at walmart for 13 dollars. You really need to get some chlorine in the water - and your ph is high, your alkalinity is high and your calcium (which you don't need in a vinyl liner) is high. I am surprised you don't have cloudy water. If the vitamin C tablet took the stain off, then it is metal - After you post the numbers I will see what is going on.