Duckweed, if left to grow unchecked, can cover your pond in a few weeks. Not only is it unsightly and will prevent you from swimming or boating, but it can also kill off any fish or other plantlife in the pond as it tends to deplete the oxygen.
Remove the duckweed manually by using a pool skimmer or similar net to scoop them out of the water. This will work best if you have a small pond, but even then, you'll probably need to employ other methods of keeping the duckweeds under control.
Cut down trees that are along the banks of the pond, particularly on the northwest and southeast sides. Fewer trees allows for more wind, which can blow the duckweed ashore where it can dry up and die. Removing trees also leads to fewer leaves dropping into the pond---leaves which provide nourishing fertiliser for the duckweed as they decay.
Install a bubble aerator in your pond. An aerator adds oxygen to the water, reducing the levels of phosphorus and nitrogen that allow duckweed to flourish.
Introduce koi into your pond before the duckweed starts to grow. Koi will eat enough of the duckweed to help keep its growth in check, but they cannot eat enough to clear away a pond covered with duckweed. So koi cannot be the only means of removing an already established duckweed problem. Ducks may also be used around a pond to eat the duckweed, but you may end up having a more serious problem with the pollutants from their droppings.
Control your duckweed with chemicals. The herbicides fluoridone and diquat are particularly effective against aquatic plants such as duckweed, and diquat is even more effective if mixed in a 2:1 ration with chelated copper.
Duckweed is actually edible by humans, if you want to try getting rid of them that way.