Vinyl Liner with Yellowish stains

[Sierra54]

I should be nearing the end of the SLAM and as the water in the deep end is clearing I can now see yellowish stains at the bottom of the transition in the deep end. I have tried vacuuming and brushing but the spots remain. I'm not sure if this could be an organic stain, metal stain, mustard algae or something else.

Here are my #'s prior to SLAM: best I can remember
FC: 0
pH: 7.8
TA: 70
CH: 175
CYA: 45
Salt: 2230

I added muriatic acid to lower the pH then started the SLAM using 12.5% liquid pool shock and have been trying to keep the FC at 18.

Any advice or insight on how to figure out what this may be? Would the liquid pool shock settle and cause this even though the pump is running 24/7? Can I trust pool store tests for metals or is there one for use at home?

Thank you.

 

[borjis]

Could be iron. My liner has some similar staining.
Put some crushed vitamin c in a sock and place it over a trouble area for a bit. See if it lifts off. If so, likely traces of iron in your fill water.