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 in acquatic environments, being very common in Lost Mangroves and Sea Breeze Cove.
If the Kongs 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.