Phylum: Chlorophyta   Family: Ulotricaceae

Spongomorpha aeruginosa  (Linnaeus) Hoek, 1963

Spongy Weed

Growing epiphytically on other seaweeds, especially Polyides. Most common in spring from the lower shore to subtidally

References: 
Fish & Fish (2011) - A Student's Guide to the Seashore;
Bunker, et al - Seaweeds of Britain and Ireland;
Brodie, et al - Green Seaweeds of Britain and Ireland