Just another thought here, your pool isn't majorly green but rather had that green patch. I can't believe that you have such a massive algae problem that it's eating the chlorine that fast. I think possibly you have some residue left in the test vial and you're not getting true results. This has happened to me some time ago. It all changed after I cleaned the vial and the magnetic stirr with some dish soap, rinsed very well and started testing again. All of a sudden my numbers were inline with my slam. You need to rinse very well all the time after testing.