Usually I only need to clean my filter a couple times a year. This year i've had to do it every 2-3 weeks. Each time when I take the filter out it's coated in algae, but the pool itself is fine. I clean the filter put it back and shock the pool, but sure enough a few weeks later it's back. I've been checking my FC and it's always in the 3-5 range and as I said other than the filter the pool is looking good. Any suggestions?