What is "gelidium"

Gelidium is a genus of thalloid red algae comprising 124 species. Its members are known by a number of common names. Specimens can reach around in size. Branching is irregular, or occurs in rows on either side of the main stem. Gelidium produces tetraspores. Many of the algae in this genus are used to make agar. Chaetangium is a synonym.