"Yomogi" is Japanese for Artemsia Princeps. It is a perennial Chrysanthemum which can be very invasive. (Thus, we have it confined in a pot, the base protected from the roots growing into the soil with a step-stone.) This herb is used to color the mochi green and give it a pleasant aroma. The "kusa mochi" (grass mochi) is enjoyed in the spring usually during Girl's Day celebrations and is cut in to a diamond shape and placed in front of the dolls.
We didn't get fancy and cut them into diamond shapes though. These we ate with "anko or an" a sweet bean paste made from boiled Adzuki beans.