Algae, the bane of ponders all over the world. I have found, however, that the string algae keeps the water crystal clear. I try and keep it under control, pulling it out by the handful when it gets too pushy, but I don't want it all to go as then I think other types of algae appear - like the one that makes the water green and cloudy. I use a plastic rake to get it off the floor of the pond and to grab floating gobs of it. When the rocks that the water runs over get too much algae, I turn off the pump for an hour and pour hydrogen peroxide on them. It works well and dilutes in the water and is ok for the fish. It also helps keep all algae down, maybe you can try some.
