I suspect it is algae, most likely a filamentous green alga, growing in clusters or small colonies. The colonies have a diameter of approximately 5–15 cm. They thrive in fast-flowing streams, particularly in waterfall areas, on steep molasse rock formations. These photos were taken in the foothills of the Alps (north side, Canton Bern, Switzerland) at an elevation of about 600 meters above sea level. Thank you in advance.