It is represented by a blue circle with a skull in the middle. The skull has a swirl of fog on its forehead.
The Fog Skull can be found on "Floodgate" on Normal or higher difficulty. To get it, you walk toward the city; look up and a Flood Combat Form will jump from one building to another above you. This Flood is the holder of the skull. To get it, you must shoot the Flood so that he will drop it. But BE CAREFUL, if you shoot it too early, it will get stuck on the roof. Simply have your sights locked onto it, and you should see it kind of crouches; it's about to jump. Quickly shoot it, and it'll land next to the sandbags next to you. The recommended weapon would be the Battle Rifle .
If you don't make it the first time, then quickly revert to the last checkpoint or restart the mission. Because this skull is early on in in the game, if you do have to restart you won't lose your progress.