Blanketweed is a term used to describe a type of fibrous algae which often grows in pond environments, and can also refer to similar hair or strand like algae's in the aquarium.
Blanketweed is notoriously difficult to control and once established will often take hold around plants, choking them to death. An uncontrollable bloom of blanketweed is often caused by excess nitrates, phosphates, or organic matter in the water and is common in new ponds or aquariums where the environment is filled with fresh, new, nutrient and mineral rich water.
In the aquarium, few algae eaters will eat this form of algae with the exception of Siamese Flying Foxes and Algae-eating shrimps. These algae eaters plus the addition of a phosphate/nitrate remover will quickly remove the algae.
In the pond, removal is a little more difficult but there are many proprietary products and methods of doing so, although their effectiveness can vary considerably.