Dried black moss, also known as fat choy, angel's hair, or hair vegetable, is a type of edible photosynthetic bacteria (blue-green algae) commonly used in Chinese and Vietnamese cooking. It has a hair-like appearance when dried with a dark color and a brittle, almost paper-like texture.
It is a popular food during Chinese New Year due to its name in Cantonese sounding the same as the phrase meaning "struck it rich" (gung hei fat choy). To prepare black moss for cooking, it must be soaked in water to rehydrate. Once rehydrated, the moss will expand and become more pliable, taking on a slightly chewy texture. The flavor of fat choy is relatively neutral and slightly earthy, and it readily takes on the flavor of sauces and soups. Fat choy has soft, supple texture that is comparable to very fine vermicelli.