Marimo is a rare growth form of Aegagropila linnaei in which the algae grow into large green balls with a velvety appearance. The species can be found in a number of lakes and rivers in Japan and Northern Europe. Colonies of marimo balls are known to form in Japan and Iceland, but their population has been declining.
The plant was named marimo by the Japanese botanist Takiya Kawakami in 1898. Mari is a bouncy play ball. Mo is a generic term for plants that grow in water. The native names in Ainu are torasampe (lake goblin) and tokarip (lake roller). They are sometimes sold in aquariums under the name "Japanese moss balls" although they are unrelated to moss.
Dummy text about Marimo Moss Balls taken from the Wikipedia page. (check it out here!)