Have light brown stains on walls of liner, won't brush off? How do I get rid of it? What is it?

Question

Never had this problem with the pool before. Chlorine level and ph are fine.

Answer 1

Knowing nothing about pools, I'm not sure if the links below will answer your question, but I believe they may.

The fourth link goes to the original page which is where I located the first three.  I included this link because there are many more questions/answers to go through -

Hope one of the answers will help you!