Phylum: Rhodophyta   Family: Plocaminaceae

Plocamium cartilagineum  (Linnaeus) P.S. Dixon, 1967

Recent genetic studies have now split this once easily identified species in 5 (3 in Britain). Common in intertidal rockpools growing on larger seaweeds and rock surfaces. Growing up to 10 cm or more, with regular sympodial branching in series, which help to identify it. (Dixon & Irvine is now out-of-date as it describes just one species.)

References: 
Bunker, Brodie, Maggs & Bunker (2017) - Seaweeds of Britain and Ireland;
Dixon & Irvine (1997) - Seaweeds of the British Isles, Volume 1 Rhodophyta, Part 1 Introduction, Nemaliales, Gigartinales