There are lots of resources online about how to keep pond water clean.  I'm not an expert on it but I do have a small pond in my backyard that I get to experiment with.  Algae is a function of nutrients and sunlight.  First, your water looks exceptionally clear.  This make your pond look pretty.  You can see fish and other wildlife easily I'm guessing 2-3 feet down.  It also makes it easy for sunlight to penetrate the surface of the water.  What is the phosphorus level of the water.  You may actually want to send in a sample.  Is the pond fed from field runoff?  Does it have high phos and nitrate levels from agricultural practices?  If you use the pond at all for recreational uses I'd look into using pond dye to color the water blue or black.  It will help reduce sunlight penetration which may help your situation.