I would question the integrity of the pool store you visited since copper will not turn a pool green. It sounds as though you were the victim of an "upsell". Although it is good to keep your pool free from metals, this is not why the pool is green. Low chlorine and high pH makes more sense. To balance the chemistry (pH, chlorine, and alkalinity), I recommend the pool also be tested for phosphates. Make sure the pool filter is clean and working properly with good flow. (Does the cartridge filter element need replacing? Is the sand filter working properly? Is the DE filter holding the DE powder?) If you have an in-ground pool with a concrete type surface with a serious copper problem, then blue or teal color stains will be visible. A vinyl liner will make it difficult to detect any "metal" type effects.