Collapsible Underwater Grass
From the Super Mario Wiki
Collapsible Underwater Grass is a type of vegetation encountered in Donkey Kong Country: Tropical Freeze. It is widespread throughout aquatic lands, namely Lost Mangroves and Sea Breeze Cove, which implies that it is able to live in both fresh and salt waters. Collapsible Underwater Grass is found in small colonies lined up on the ground underwater.
If the Kongs simply swim into a patch of Collpasible Underwater Grass, it will retract and reveal common items such as bananas. The Prima guide for the game suggests using a corkscrew move to collapse the grass, which is not necessary. If the Kongs leave a patch of Collapsible Underwater Grass, it will quickly return to its initial position after a few moments.