Phylum: Rhodophyta   Family: Hapalidiaceae

Melobesia membranacea (Esper) J.V. Lamouroux, 1812

Epiphytic on several seaweed species, especially Furcellaria and Laminaria. Occurs in the mid to lower intertidal and sub-littoral. Very common

References: 
Wells (2007) - A Field Guide to the British seaweeds;
Irvine & Chamberlain - Seaweeds of the British Isles, Volume 1 Rhodophyta, Part 2B Corallinales, Hildenbrandiales