Helping son close pool that is only 2 years old as installed in fall 2017 so 2nd full season of operation . Water is clear . First time he closed as previously the installer closed / 27500 vinyl lined inground with filter plates and is a SWG . Readings look good as PH 7.5 FC 6.0 CC 0.0 TC 6.0 , Akl 90 and Hardness 170 and CYA 50. Lowered water down the returns and notice the dark tile top section of the vinyl liner looked cloudy or faded and it appears to go all the way down on the pebble sections . Used a magic eraser and it lighten it but no means is it back to a brilliant dark upper section. Eraser has some dirt on it after the scrubbing . Thinking we need to clean the liner or add something to the water before closing . With water chemistry pretty good am confused by what it is and what to do , HELP in Eastern PA as winter is approching