What is "fat choy"

n. a blue-green alga ((taxlink Nostoc flagelliforme species noshow=1)) used as a vegetable in Chinese cooking

Fat choy ( Nostoc flagelliforme), also known as faat choy, fa cai, black moss, hair moss or hair weed is a terrestrial cyanobacterium (a type of photosynthetic bacteria) that is used as a vegetable in Chinese cuisine. When dried, the product has the appearance of black hair. For that reason, its name in Chinese means "hair vegetable." When soaked, this vegetable has a very soft texture which is like very fine vermicelli.