Weird stains on liner

[Rangerman]

...it looks like just a standard 'needs brushing' appearance. The liner is mostly a sky blue so it really shows up. A friend said he had something similar a few years ago. His installer said it was behind the liner and was 'showing through'.
Anyone ever had a similar experience and can it be removed safely?
Thanks for the replies in advance.

Rangerman
Arkansas

 

[JasonLion]

You might as well double check to see if they are iron stains, even though it doesn't sound like it. Try placing a vitamin c tablet on one of the stained areas and see if it clears up in that spot.

The behind the liner thing is very difficult to treat, if that is what it is.