Mung bean vermicelli are very thin noodles made with mung bean starch. Small bundles of noodles 50 grams each are sold in bags. Once cooked they are super-thin and almost transparent. You can use other Asian rice vermicelli as a substitute. Korean dangmyeon starch noodles are too thick to substitute.
Look for them in the noodle section of Korean grocery stores. After opening, seal the package well and store in a cool, dry place.
One bunch of mung bean vermicelli noodles