"Mozuku," dark brown seaweed. もずく. Nemacystis decipiens.
This type of seaweed is found in Okinawa.
Eaten as a vinegared side dish (sunomono) at home. Preserved in salt, it can also be eaten as a snack (tsumamimono). Finally, mozuku is powdered and used in health supplements.
As sunomono, the sour taste can be rather overpowering. Various healthy properties are ascribed to this type of seaweed, so it is quite popular. Mozuku is sold "ready to eat" in supermarkets in small plastic containers, with the vinegar dressing already added.