What question does the littlun with the birthmark raise?
When the boys come down from the mountain, having finished their exploration, Ralph calls another meeting and they begin to discuss their situation. They point out that they are on an island and that they can hope for a rescue but it might be some time. Jack is confident that the island can provide everything they need so he projects an air of confidence but Ralph wants to get organized so that they can better hope for a rescue.
At some point, a group of younger boys urges one of their number to come forward to describe something he had told them about. The boy with the mulberry colored birthmark hesitantly asks the older boys what they are going to do about the snake-like beastie thing that he saw the night before. He claimed that it came out of the dark and went away and then came back again and that it wanted to eat him.
The older boys laugh it off and suggest that it would be impossible on an island like that but it introduces an element of fear and uncertainty that will eventually overwhelm the boys.