water milfoil vs aquatic plant

water milfoil and aquatic plant both are nouns.

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

As nouns, aquatic plant is a hypernym of water milfoil; that is, aquatic 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
aquatic 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 hydrophyte, hydrophytic plant, water plant.

water milfoil (noun)	aquatic 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