Your system is aging normally. Algae spores will find their way into your system over time, Its unavoidable. It is part of a healthy ecosystem. I find its easier to embrace it, rather than overreact when they enter your system. I admit they can be incredibly frustrating at times. But usually with alittle patients they will cycle through your system on their own. Just stay diligent with your system maintenance, and your system will blossom beautifully. Also just because your system has a lot of algae, it doesn't mean the system is unhealthy. My corals seem to be happiest when my system is nutrient rich with plenty of algae. IMHO this is the key to long term success with a reef.