I had drained all but a foot of my 12'x 42" round above ground pool. The remaining water was CLEAR, though there was a small amount of remaining "dust" on the bottom of the pool. Thinking that once it was full, the filtration system would capture remaining silt. To my SHOCK, once 3/4 full, it was a DEEP MOSS GREEN! I filled it out of the hose, like I have several times, and I'm stumped. WHY would the water out of our hose turn the pool GREEN?
If you get a bag or 2 of shock treatment it will help knock the algae out. This time of year I find the toughest to keep my pool clean as the water is warming and it gets away fast shock treatment brings the chlorine levels up. The link is for a small box of bags I usually loose control several times a year when I forget to put a chlorine tablet in the skimmer after all the kids and grand kids are over. If you treat it quickly it will only take a day or 2 to clear up if you let it wait a week it may take a week or more to clear up. Vacuuming the bottom and sides will also help reduce the growth. If you don't have a test kit these test strips are very easy to use to track the chlorine and PH levels of the water. Muriatic acid and baking soda are used to raise and lower the PH. A proper PH level will also help to stabilize the water. The last thing I use is blue clarify. This helps the water to be crystal clear and sparkly 1 bottle usually last an entire year as it only takes a little and I only use it if the water starts to appear less clear. Last make sure your filter is clean back wash or clean the filter elements you don't see all the gunkies growing in there and it is a great breading ground if not clean. Hope this helps and you get to enjoy your pool.