Pond algae is a type of plankton, a microscopic plant. Algae become a problem when they grow in large numbers and then "bloom," or become visible because of slightly increased size and greatly increased numbers. Algae can form mats that float on top of a pond or fill the water more uniformly, turning it a dark green or black. Once a pond has an algae bloom, there can be an unpleasant smell, and the algae can remove enough oxygen from the water to kill most fish. Algae blooms usually occur in late summer when the water is very warm--this is the time to harvest. Harvested pond algae can be composted.
Wait until your pond algae has risen to the surface of your pond in a large mat or large clumps.
Skim the algae from the surface of the water with a large fine-mesh net or with hardware cloth stretched between two poles. Turn the net inside out frequently and hose off with running water to remove the harvested algae. Store the algae far enough from your pond that it will not be washed back into the water. Your compost pile can be a good place for it.
Walk out into the pond wearing hip waders if the pond is shallow or take a small boat onto the pond if it's of sufficient size and use your net to skim algae that cannot be reached from the shore.
Wait one week and recheck your pond for additional algae growth. Skim again if necessary.