Many plants that are considered to be invasive weeds are beautiful. Duckweed is usually classified in the Lemnoideae family. Here is a description of the plant from Wikipedia: Lemnoideae:
These plants are very simple, lacking an obvious stem or leaves. The greater part of each plant is a small organized "thallus" or "frond" structure only a few cells thick, often with air pockets (aerenchyma) that allow it to float on or just under the water surface. Depending on the species, each plant may have no root or may have one or more simple rootlets.