From Halopedia, the Halo wiki
On the three floors of the city's detention center there are what looks like small, yellow insects crawling around the place. The pests seem to slide on the floor and on the walls, and once anyone approaches one, it slides away. The origin of the pests is unknown, but they may be Scrub Grubs that live onboard Covenant ships and feed on machinery, as they are as abundant as they were said to be.
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