Seaweed
From Starfy Wiki
Seaweed (Japanese: 海草) is an object that appears in Gluglug Lagoon underwater. It appears as a large green leaf, that folds over when hit by a Star Spin. When folded over, it may reveal nothing, empty bubbles, Pearls trapped within bubbles, a Treasure Chest, or a hidden door. Lighter green Seaweed can be found in Skydye Heights.
Seaweed like the ones in Gluglug Lagoon can also be found in the Stage 9-4 (Pufftop) boss battle with Konk, where Konk and a fake Konk hide inside the Seaweed. This seaweed can only be turned over if Konk isn't in it or the player hits the real Konk hiding in the seaweed.