Seaweed
"Seaweed" is a colloquial term and lacks a formal definition. A seaweed may belong to one of several groups of multicellular algae: the red algae, green algae, and brown algae. As these three groups do not have a common multicellular ancestor, the seaweed are in a polyphyletic group. In addition, some tuft-forming bluegreen algae (Cyanobacteria) are sometimes considered to be seaweed.
Caulerpa taxifolia
Caulerpa taxifolia is a species of seaweed, an alga of the genus "Caulerpa" native to the Indian Ocean. It is widely used ornamentally in aquariums, because it is considered attractive and neat in arrangement, and is easy to establish and care for. The alga has a stem (rhizome) which spreads horizontally just above the seafloor. From this stem grow vertical fern-like pinnae, whose blades are flat like those of the yew ("Taxus"), hence the species name "taxifolia".