star-duckweed vs duckweed

star-duckweed and duckweed both are nouns.

Word	Noun	Adjective	Verb	Adverb
star-duckweed	Yes	No	No	No
duckweed	Yes	No	No	No

As nouns, duckweed is a hypernym of star-duckweed; that is, duckweed is a word with a broader meaning than star-duckweed:

star-duckweed: cosmopolitan in temperate regions except North America
duckweed: any small or minute aquatic plant of the family Lemnaceae that float on or near the surface of shallow ponds

star-duckweed (noun)	duckweed (noun)
cosmopolitan in temperate regions except North America	any small or minute aquatic plant of the family Lemnaceae that float on or near the surface of shallow ponds