Phylum: Chlorophyta   Family: Cladophoraceae

Cladophora rupestris (Linnaeus) Kützing, 1843

A dark green (sometimes bluish) species, multi-branched and matted giving a coarse feel. Very common on rocks and in tidal pools found in the lower tidal zone to sub-tidally.

References: 
Brodie, Maggs & John (2007) - Green Seaweeds of Britain and Ireland;
Bunker, Brodie, Maggs & Bunker (2017) - Seaweeds of Britain and Ireland