Hildenbrandia rubra is thin and tightly packed to the substrate, forming a continuous sheet on rocks and hard to remove except by breaking the rock.  It has a leathery consistency, with a smooth surface but is sprinkled by small elevations that correspond to conceptacles.
This dark red almost black algae grows quite slowly, and it builds patches that can be quite widespread; sometimes it can be difficult knowing if you are looking at one large individual or several small.