Green onion (scallion) contains sulfur compounds and flavonoids similar to bulb onions, but in lower concentrations. People use it fresh and uncooked to add flavor and color. Along with other alliums, it has antioxidant and antimicrobial properties, though its main draw is the small amount of vitamin K and vitamin C you get for almost no calories.
There is no established use of green onion as a concentrated supplement. Its function is almost entirely as a fresh food ingredient.