duckweed
/ˈdʌkˌwid/
noun
- A very small plant that floats on the surface of still or slow-moving water, often covering ponds and lakes with a green layer.
- The pond was completely covered in duckweed, making the water look like a green carpet.
- We had to skim the duckweed off the top of the aquarium to let more light reach the fish.
- Duckweed grows so quickly that it can double its mass in just a few days.