I've actually struggled with this issue going on 2 years now. I have an inground pool with vinyl liner, 18 x 36. I have a mysterious light brown stain that appears randomly throughout the pool. I don't believe its an algae but the local pool store tells me there isn't an abundance of metal in my water. I've used United Pool Stain Treat and it will remove the stains temporarily, but they return after a couple days and that product creates a gummy white material that clogs my DE filter. My water is crystal clear, but the overall appearance is horrible because of this staining. Here are my chemical amounts. Chlorine 1.5, TA 185, PH 6.8 CYA 30. I'm also not sure why I can't get my PH and Alkalinity to balance out.

Thanks for the help