Generally, if your pool turns green, it is because of algae. While algae is pretty much always present to some extent, it can become resistant to normal levels of chlorine, and if the conditions are right, it can take over, in a very short period of time.
1. Low chlorine levels;
2. Phosphates; and
3. Warm water.
If you already have low chlorine and phosphates present in your pool, then adding a solar pool cover (which warms the water) will most certainly help existing algae to thrive.
So while a solar cover won’t actually ‘turn your pool green’, it will warm your water by up to 8 degrees, so if the other conditions are right, adding a solar cover can easily accelerate algae growth, very rapidly.
You need to get the water balance in your pool right before putting the cover back on. Take a water sample down to your pool shop, explain what is happening and they will work out what you need, based on your pool water condition, and set you on the right path. If you wish to do it yourself, (and it is only just starting to turn green), the first step is an algae starver. This will remove the phosphate build up, (the algae food), and thus, the algae starves. Filter your pool for 12-24 hours, to remove all the algae spores from the water.
Its also a good idea to backwash or clean the filter afterwards, to ensure there are no algae spores trapped inside the filter. If there is algae spores left in the filter, you have a greater chance of the problem recurring.
If the algae is really severe (ie you can see it on the pool walls and floor), you really wont be able to avoid a trip to the pool shop. Superchlorination is required, and you will need professional advice with regard to quantities and concentrations of chemicals. Ensure your pool cover is completely removed when you superchlorinate, and do not put it back on the pool until chemical levels return to within the Australian Standard.