One reason is a low alkalinity level. It is a good idea to keep the alkalinity level between 120-150 ppm which is the high end of the ideal. You should be also checking your pH and alkalinity levels at least once a week to be sure that your water is within the proper ranges. pH must be between 7.2 and 7.8 to prevent damage to your spa and or your body. Green water could also mean there are minerals in the water. These can be cleaned by using Protection Plus. Green water could be sign of algae in the water.