Fa Cai which is also known as black moss, hair weeds; hair moss; Nostoc flagelliforme, a cyanobacterium (type of photosynthetic bacterium) used as a vegetable in Chinese cuisine which, when cooked, resembles black human hair.
In Hongkong, Black moss is widely eaten during Chinese New Year, mainly because its Chinese name, Fa cai, is homonymous with "prosperity” in Cantonese and Mandarin.
Fa refers to hair and Cài means vegetable, greens in Chinese.
Fa Cai is usually eaten during the Chinese New Year season because its name sounds like the Mandarin wor F?cái which means "get rich, make a fortune, make a pile, coin money, earn good money”
In Cantonese Fa Cai is known as Fat Choy.
Personal Note: I encountered this Chinese food while watching a Television Show in Germany entitled Food Hunter hosted by Mark Brownstein who is a pioneer: a latter-day explorer, who scours remote corners of the world in pursuit of culinary curiosities.