Algae is why your pool is green.

Almost always algae. Not a chemical imbalance, not a filter problem (though that can make it worse). Just free-floating algae that got a foothold because your chlorine level dropped or your pH went wonky.

The quick fix is shock—superchlorinate to kill everything in the water. Use calcium hypochlorite or a non-stabilized shock if you can; it works fast. Follow the dose on the bag, don’t guess. Run your pump 24/7 and brush the walls and floor to break up any algae clinging to surfaces. Vacuum to waste if you have that option (sends the dead algae straight out, not back through the filter). Add a clarifier the next day if the water is still cloudy—that’ll clump the dead stuff so the filter catches it.

The two things that cause green pools most often: not enough chlorine, or high phosphates (algae food). If you clear this one and it keeps coming back, get your phosphate level tested. Otherwise, just shock, brush, filter, wait.

Don’t overthink it—shock it and scrub.