Phylum: Ochrophyta   Family: Sphacelariaceae

Sphacelaria cirrosa (Roth) C. Agardh, 1824

Small Brown Feather Weed

A littoral seaweed, usually epiphytic on Halidrys siliquosa or Cystoseira baccata but sometimes adrift as small balls in intertidal pools. Common but infrequently recorded

References: 
Newton (1931) - A Handbook of the British Seaweeds