From The Aquarium Wiki
Blanketweed (also know as string-algae) is a floating filamentous algae that typically populates ponds and can be very prolific covering the surface of the pond obscuring the owners view of the fish.
It grows because of excessive plant nutrients in the water and is considered ugly. However blanketweed is not harmful to the pond as it oxygenates the water, absorbs minerals and reduces unwanted ammonia, nitrite and nitrate levels.
The use of an algaecide or water changes after manual removal will temporary prevent it covering the pond surface in the near future. But if the main cause of the excess nutrients is not removed then the blanketweed will return.