From Wikipedia

Killer Algae
Caulerpa taxifolia

Caulerpa taxifolia is a species of seaweed, an alga of the genus Caulerpa. Native to the Indian Ocean, it has been widely used ornamentally in aquariums. The alga has a stem which spreads horizontally just above the seafloor, and from this stem grow vertical fern-like pinnae, whose blades are flat like yew, hence the species name "taxifolia" (the genus of yew is "Taxus"). The alga produces a large amount of a single chemical that is toxic to fish and other would-be predators. This is in contrast to other plants which produce a variety of toxins, but in reduced amounts.

It is one of two algae on the list of the world's 100 worst invasive species compiled by the IUCN Invasive Species Specialist Group.