Test your chlorine tonight after the sun is off the pool, and again in the morning before sunup and compare the two numbers. If you don't lose any chlorine overnight, then odds are you're not fighting algae and just need to do some more filtering. (What's your CH level, and what do you use to chlorinate?) If you do lost chlorine overnight, then it's likely algae trying to happen and you need to shock the pool and keep it there until you don't lose chlorine overnight.