Phylum: Ochrophyta   Family: Chordariaceae

Elachista scutulata (Smith) Areschoug, 1843

A common filamentous alga that grows epiphytically on Himanthalia elongata

References: 
Fletcher (1987) - Seaweeds of the British Isles, Volume 3 Fucophyceae (Phaeophyceae), Part 1