duckweed: any small or minute aquatic plant of the family Lemnaceae that float on or near the surface of shallow ponds
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
cladonia: type genus of Cladoniaceae; lichens characterized by a crustose thallus and capitate fruiting bodies borne on simple or branched podetia