From your pics I will hazard that you are hosting the much discussed old often cussed FA. Filamentous algae. Check many other posts here on control. I usually use google. Try " pond boss algae" and you should get lots of options.
Get a single strand, put it on a light colored background (like computer paper) and take a relatively close well focused picture, with something in the picture so we can get an idea of the scale of the plant.
You can also look here and see if anything looks familiar.
