A Stain That Comes and Goes - Metals or Something else?

I have a light brown stain all over a vinyl liner including the walls. I throw in some PH down and the stain breaks up before my eyes. I run the barracuda and it looks great. I rebalance my water and a few days later the brown stain is back again. I have also used meta ban and it breaks the stain up just as the PH down does. However, it comes back again as notes when I rebalance the water. Puzzled as to what to do next. Can anyone help?

Some people here have had luck removing metals from their pools with a CuLater - sort of a teabag that goes into the skimmer and is supposed to trap metals dissolved in the water.