water milfoil vs water plant

water milfoil and water plant both are nouns.

Word	Noun	Adjective	Verb	Adverb
water milfoil	Yes	No	No	No
water plant	Yes	No	No	No

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

water milfoil: an aquatic plant of the genus Myriophyllum having feathery underwater leaves and small inconspicuous flowers
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 milfoil include aquatic plant, hydrophyte, hydrophytic plant.

water milfoil (noun)	water plant (noun)
an aquatic plant of the genus Myriophyllum having feathery underwater leaves and small inconspicuous flowers	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