Phylum: Chlorophyta   Family: Ulotricaceae

Acrosiphonia arcta (Dillwyn) Gain, 1912

Arctic Sea Moss

A dark green thallus, mat or tuft forming on rocky surfaces. The fronds are single-cell wide with rounded tips. The habitat is rockpools in the middle to lower shore

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