Phylum: Ochrophyta   Family: Chordariaceae

Asperococcus fistulosus (Hudson) W.J. Hooker, 1833

Thin Sausage Weed

A clump-forming species growing from a discoid holdfast, epiphytic on larger seaweeds such as Fucus. Found in pools at low water and sub-tidally

References: 
Bunker, Brodie, Maggs & Bunker (2017) - Seaweeds of Britain and Ireland;
Newton (1931) - A Handbook of the British Seaweeds