It is also often found on waterfalls. Filemantous algae, although unsightly in appearance, does have some redeeming qualities. Filamentous algae is also an excellent food for pond fish, containing some important vitamins and color enhancers. Of course, as your question indicates, if filamentous algae gets out of control it can cause problems. In particular, small fish can get tangled in the algal mass and suffocate.
Filamentous algae can also strangle desirable pond plants.
The simplest and safest way to rid your ponds of filamentous algae is to mechanically harvest it with a stick or rake. This is not much of a chore in most backyard ponds. You can then slow the return of filamentous algae by shading your ponds or adding more plants that will compete for nutrients in the water. Water lilies accomplish both goals simultaneously, but they would have to cover at least 50 percent of the pond surface to have a noticeable effect. A good crop of water hyacinth will also remove nutrients rapidly.