Covenant pest
From Halopedia, the Halo wiki
Template:Ratings There are Covenant Pests that can be found in Halo 2 on the level "Gravemind". On the three floors, two of which contain the detention blocks, there are what looks like small insects crawling around the place. No one really knows how and where these little pests came from but some theories are that they are a food source. They came from plant life that the Covenant brought from Installation 04 or certain shipments from other planets brought them through storage boxes. The pest is yellow and seems to slide on the floor and on the walls, and once anyone approaches one, it slides away. They are able to be shot at, but cannot die.
Other wildlife on High Charity include what appear to be fish in the High Charity gardens.