Phylum: Chlorophyta   Family: Chaetophoraceae

Stigeoclonium lubricum (Dillwyn) Kützing

Relatively common in flowing clean water, usually attached to the previous year's plant-stems

References: 
John, D.M., Whitton, B.A. & Brook, A.J. (2011) - The Freshwater Algal Flora of the British Isles