water star grass vs water plant

water star grass and water plant both are nouns.

Word	Noun	Adjective	Verb	Adverb
water star grass	Yes	No	No	No
water plant	Yes	No	No	No

As nouns, water plant is a hypernym of water star grass; that is, water plant is a word with a broader meaning than water star grass:

water star grass: grassy-leaved North American aquatic plant with yellow star-shaped blossoms
water plant: a plant that grows partly or wholly in water whether rooted in the mud, as a lotus, or floating without anchorage, as the water hyacinth

Other hypernyms of water star grass include aquatic plant, hydrophyte, hydrophytic plant.

water star grass (noun)	water plant (noun)
grassy-leaved North American aquatic plant with yellow star-shaped blossoms	a plant that grows partly or wholly in water whether rooted in the mud, as a lotus, or floating without anchorage, as the water hyacinth