GREEN OF YELLOW WATER

WHAT TO DO IF THE WATER IS GREEN OR YELLOW?

It's biofilm.

You will need to test for phosphates and remove them and granular chlorine, lithium or brome shock with a 60% algaecide.

Then, decontaminate.  IE: put any accessory or floating item such as a mattress, in the pool overnight, that has been in the pool in the last 3 weeks.  Wash all bathing suits, towels, flip flops, etc.  One single algae left on any item will gladly enjoy your pool and you'll have to treat it a second time.