While waiting for others to respond:

Have you doublechecked your cyanuric acid level and if it is actually zero why do you have no cyanuric in an outdoor pool? This is sold as stabilizer or conditioner and should say cyanuric acid as ingredient. If cyanuric levels are zero chlorine is quickly destroyed by sunlight. This is also why it is often recommended to add chlorine in the evening, though if you need to add during the day to make sure levels don't drop you should do so. I suspect others are going to say that 8.5 free chlorine is not high enough for an algae problem. Hopefully others will come in and give advice for amounts and proper chlorine levels given your ph etc.