When with classify a person or group as "other", we define ourselves in opposition to that person or group. In other words, we believe that the Other is what we are not. So, if we truly see a person or a group as an Other, we do not believe that we can understand the group/person, or that we have anything in common with the group/person.
What does Marlowe say about the residents of the Congo that indicates this type of belief? How can you tell that he does not understand them/believe that he can understand them? How can you tell that he considers himself fundamentally different from them?
I would definitely examine the portions of the text where Marlowe views people from the deck of his boat. He does not talk to them, but observes and comments on them. I would also recommend that you take a look at his comments about the boilerman on his ship.