Duckweed is high in protein and fats, and it is a nutritious food source (and an excellent natural laxative) for vegetarian fishes such as goldfish and carp; the common name comes from the fact that ducks are very fond of eating this plant.
I got it with a bag of fish many years ago, and it has been excellent in my 10g pygmy cory spawning tank. It has appeared in other tanks from water changes tank to tank, but I have never goound it a problem as it is easy to net out the few plants each time. I wold rather see plants like this thriving than not have them.