From Creatures Wikia
Gumin Grass (Poa guminum)grass, the Gumin Grass is a chief food source for many residents of its home terrarium, such as the nudibranch and fish species. Naturally, it can only grow and survive in salt water. It releases green, floating seeds when grown to full size.
At the beginning of the game it can be found in both halves of the Marine Terrarium - however, in the left side, the nudibranch often preys it to extinction.