The first reference to a mask in the story is in chapter 4, when Jack sees his own face in the reflection of the pool. He has just smeared his face with clay and charcoal, and laughs excitedly as he sees, instead of his normal face, "a mask." This point in the story marks the beginning of Jack's transformation into the monster he later becomes. With the clay and charcoal mask, Jack's "laughter (becomes) a bloodthirsty snarling." The mask is also described as "a thing on its own, behind which Jack hid, liberated from shame and self-consciousness." In other words, with the mask to hide behind, Jack felt liberated to exercise his more primitive, animalistic, uncivilized impulses.
In chapter 10, Jack insists that his followers had, the previous night, beaten but not killed the beast. We know of course that the so-called beast was in fact another of the boys, Simon. Jack, however, says that it was the beast in disguise. The disguise in this scene is a symbol of Jack's denial, and of his own mind's refusal to acknowledge the horror of the murder of the night before. It is also potentially a ready-made excuse for Jack to kill anyone else he wants to later on. He can always insist that the victim is in fact the beast in disguise.