This plant is a variant of Ludwigia inclinata var. verticillata, with spiralling submerged leaves growing very densely. It is probably a mutant of L. inclinata var. verticillata ‘Cuba’ that was found in a Vietnamese aquatic plant nursery.
Typically, one plant will have many stems, creating the look of multiple plants. Hornwort lacks true roots, but certain leaves function to help anchor the plant in the substrate. It may also grow rhizoids (hair-like roots) to help anchor the plant.