Flutterheart Grass is a Key Item in Xenoblade Chronicles 2. It can be obtained at the Sealed Quarry in the Kingdom of Tantal after starting the DLC Quest Making Love Source, which is part of the New Quests Pack 3 of the Expansion Pass.
Flutterheart Grass is one of the ingredients needed to make Love Source at the Fiercest Fire. Only one Flutterheart Grass can be in the inventory at any one time. After the quest is completed, it will respawn at the Sealed Quarry, and will continue to do so every time it is used to make another Love Source.