Subwassertang
Süßwassertang.
That means "freshwater seaweed" in German. This is a very
rare liverwort (probably not even a liverwort) that is yet to be positively identified. The specimens I have are supposed to have originated from the Botanical Gardens in Germany. Some people call it Fern Gametophyte, among many other common names. Grows like Pellia (Monoselenium
tenerum) but is larger and rounder. As you can see in the picture,
the growth is very full and can eventually get into a nice tight bunch.
Under low light conditions (and possibly without CO2 injection), the plant does not grow as tight or round, although it can still survive well even in the dimmest corners.