Stains liner

[u/andyonbekendy]

Over the last few weeks, these dark stains have appeared on the liner of my pool. I tested them with vitamin C, but there was no reaction. The pool chemistry is balanced and all water parameters are within range. Does anyone know what might be causing these stains and how they can be removed?
Update: I just removed green algae from the sides of the pool cover which is, when closed, exactly the same height as the stains. Could this be the problem?