Phylum: Cyanobacteria Family: Oscillatoriacea
Lyngbya majuscula Harvey ex Gomont
Forms mats 2-3 cms across either anchored to rock or free-floating. Fairly common the the upper tidal zone
References:
Newton (1931) - A Handbook of the British Seaweeds;
John, Whitton & Brook (2011) - The Freshwater Algal Flora of the British Isles