It is the hornwort (Ceratophyllum) gently wriggling or moving from side to side.
Any of about 100 species of small bryophyte plants belonging to the phylum Anthocerophyta. Unlike liverworts but like mosses, hornwort sporophytes have stomata. The hornwort gametophyte consists of a low thallus, out of which numerous slender, upright sporophytes tipped with sporangia grow. The sporophyte has a meristem that elongates the sporophyte with new growth, a feature that distinguishes the plant from the other bryophytes. The name of the hornworts was suggested by the hornlike appearance of the sporophytes. See more at bryophyte.