Duckweed is also known as Lemna minor.
These small bits of green (one sixteenth to one quarter of an inch across) can blanket a pond quite quickly.
That is unless there are fish present who consider it the salad part of their diet. I used to keep an aquarium full of the stuff just to feed the fish as they would never let it get out of hand. Quite hardy to zone 3, it is found in almost all northern waters.
If you’re purchasing other pond plants, you’ll often find it has come along as a hitch-hiker.
Otherwise a single handfull will populate most ponds quite quickly (as long as there are no large fish there to eat it).
Ivy Leaved Duckweed (Lemna triscula)
This species stays just below the water surface. Other than that, the fish love it too. Equally hardy floating oxygenators to the regular form.