hmmm..interesting vinny. until this past winter, my pool was uncovered all winter. this past winter, i put on a leaf net, but no other cover..i'm trying to think of anything else i've done different..solar cover last year for most of the pool season? i was thinking maybe it was the amount of chlorine i used..i dumped in about 10 jugs in 4 days to shock, but i'm sure i've done that before. i knew these marks weren't algae persay by the way because they weren't slimy at all to touch..like a true mild green stain on the blue liner..just looks kind of like the pool walls are dirty, you know? well, tomorrow afternoon, we'll see what the magic brush does..if that doesn't work, i think i'll have to get some vitamin C to eliminate metal staining (which i think is highly unlikely given i use a chem test kit and we're blessed here with our water in that my chemicals are great..so you know, i did start with some cya in my pool this season, but did add more, but surprisingly, didn't need much..about 3/4 of one one quart size bottle or so brought it up to the 60 i was looking for. last year, i had to dump in 2 or 3 of those to get the level up. i'm wondering if maybe the cya was part of the problem..to simplify the differences, year before last i had nothing but a heater, pump, filter and used chlorine and chlorine pucks in combination. no cya, no solar cover, no salt generator. and no stains. ph and alkaline have always naturally been a good level.