Rtas 'Vadum in crashed Flood ship
From Halopedia, the Halo wiki
When you jump down the hole into the Unnamed Flood-Captured Battlecruiser's Crashsite, there will be a couple of dead Ultra Elites lying around. Sometimes, one or more will have Rtas 'Vadum's face, in which two mandibles are missing. You can get it infected, too, if you push it down the hole and shoot some Flood Growth Pods to release Infection Forms. When the Elite turns into a Flood Combat Form, his armor remains silver, but changes to default blue -- like all infected Elites -- when the body is destroyed.