Sea worm
From Halopedia, the Halo wiki
Sea worms are viewable creatures that can be seen in the shallow waters of the Silent Cartographer. They are very small, and wriggle around near the surface of the water. You can see these worms if you walk into the ocean around this level, until you are submerged in water but still in shallow water. They are harmless to the player, and they do not interact with your character at all.
