A polysaccharide (a big sugar molecule) coming from the edible fruit bodies of the Silver Ear mushroom in China. Legend has it, imperial Concubine Yang Kuei-fei - the most beautiful woman in Chinese history - used it for her facial care.
So Tremella Extract is a big molecule (more than 1M Da molecular weight) with sugar constituents of Mannose, Xylose, and Glucuronic acid (20%). The last one is the most important one, as Glucuronic acid is also a major component in hyaluronic acid (HA). Thanks to this, Tremella is claimed to be an awesome moisturizer with slightly greater water-holding capacities than HA itself. This is quite a big deal as HA is known for its crazy water-holding capacity (up to 1000 times its own weight).
Not only that but Tremella also seems to have some antioxidant effect, it's not sticky (humectants do tend to be sticky) and it leaves skin moist and smooth. No wonder, Concubine Yang Kuei-fei liked this mushroom.