Looks like good ol' FA (filamentous algae) to me. You can kill it with Cutrine Plus (copper sulfate), or there are various other aquatic herbicides available that will also do the trick, and likely have less potential for negative side effects (copper sulfate can cause a build-up of undesirable metals in the pond which are harmful to the fish).
Your simplest solution, though, if the FA is only growing around the edges, might be to fertilize the pond with a commercially-available liquid pond fertilizer. Fertilizing will probably briefly make the FA grow more, but within a week or less you'll get a good plankton bloom that cuts out the sunlight penetrating the water, and the FA will die for lack of said sunlight.
And, a not-insignificant side benefit is that your fish will grow markedly faster. Most pond owners who fertilize do so mainly just for this reason, because plankton are the base of the food chain and when you feed them you basically turbo-charge the entire food chain such that your fish will grow drastically faster and larger.
