Conferva.
Botanical name:
Conferva sp. Confervae.
Green cakes are made of the slimy river confervae in Japan, which, pressed and dried, are used as food.
Sturtevant's Edible Plants of the World, 1919, was edited by U. P. Hedrick.
A herbal distance course
Henriette's herbal is one of the oldest and largest herbal medicine sites on the net. It's been online since 1995, and is run by Henriette Kress, a herbalist in Helsinki, Finland.