[Hair Moss; Fat Choy = "Hair Vegetable"; Nostoc flagelliforme]
Not actually a moss, this land dwelling cyanobacteria forms long strands that look like hair. It is harvested in the Gobi Desert and the Qinghai Plateau, but harvesting has been restricted due to resulting erosion. Due to increasing cost, sellers have responded in the time honored Chinese way - by adulterating the product. Real Fat Choy is dark green - the adulterant strands, made from starch, are usually black. Real Fat Choy will stand up to over 30 minutes of simmering, but adulterant strands will disintegrate.
The photo specimen skein was purchased from a large Asian market in Los Angeles. It was 16 inches long and folded over, weighed 3-5/8 ounces (100 grams) and smelled like drying grass. Individual hairs are no thicker than human hairs, but are actually made up of many microscopic strands of cyanobacteria.
More on Algae and Cyanobacteria.
Fat Choy appears mostly in Buddhist and Cantonese cuisines. Why Cantonese, which is in the south, while this stuff comes from the north? Because the Cantonese are reputed to eat anything and everything that won't kill them, and a fine selection of things that will. Due to an accidental pronunciation similarity, Fat Choy has become associated with gaining a fortune and is served during Chinese New Year and other celebrations.
Fat Choy has only a vague grassy taste of its own, but is supposed to absorb flavors from the broth it is cooked in, as bean thread noodles do. I recommend sticking with bean threads and not eating this stuff due to desertification resulting from its harvest, possible risks to health (see below) and lack of redeeming culinary value.
Buying & Storing: I have only seen this in the San Gabriel Superstore, a very large Asian market in Los Angeles. It will keep at least a year in a sealed airtight package.
Cooking: Fat Choy is soaked before cooking, and then used similarly to bean thread noodles. I've found that even after an hour of soaking it still takes about an hour of simmering to at least partially soften its hair-like texture. Beyond that, an additional hour has little effect. It tends to mat together in a tight tangle that's difficult to impossible unravel.
Health & Nutrition: A modest amount of research has been done on this item. In China it is popularly thought quite nutritious, but laboratory analysis at the Chinese University of Hong Kong found it to be entirely indigestible and of no nutritional value. Beyond that, it has been found to contain toxic amino acids that could negatively impact normal nerve cell function, possibly leading to degenerative brain diseases and dementia. It is, however, generally used in very small quantity, so you probably won't get Alzheimers or Parkinsons from an occasional New Years treat.