Phylum: Cyanobacteria Family: Microcystaceae
Gloeocapsa atrata Kützing
Colonies variable in size depending on age and, maybe, habitat. Widespread on various substrates and not necessarily acidic
References:
John, D.M., Whitton, B.A. & Brook, A.J. (2011) - The Freshwater Algal Flora of the British Isles