It is made from the leaves of yomogi — aka Japanese Mugwort.
Uses including mixing it into dough for mochi, noodles, steamed dumplings, etc.
Finished mochi are also sometimes rolled in the powder for a decorative, flavourful coating.
The powder is not the same as the dried leaves; they are quite fibrous.
You may need to soak the powder in water before using, depending on how it will be used in a recipe.
30g fresh yomogi leaves = 10g of powder