I remember reading a young adult novel in the '90s about a girl traveling on a spaceship. If I recall correctly, there's a dangerous race of aliens who can disguise themselves as humans, and she discovers that one of them is on the ship with her. She somehow ends up inside the walls of the spaceship, I believe in order to spy on the alien, and I think it ends up pursuing her through the walls toward the end of the book. Anybody know what it could be?
I think this might be Alien Secrets by Annette Curtis Klause.
The basic story follows a young girl on a spaceship, named Puck, traveling to meet her parents. She befriends an alien on the ship, who was going to the same planet but also looking for a artifact stolen from him, and they discover smugglers, other stolen artifacts, another alien disguised as a human, and ghosts.
There is a dangerous race of aliens, and one is on the ship with them, disguised as a human, which matches your description. The human disguise is specific to that individual, though, there isn't a mention of others of the race being disguised like that...but between her disguise and the fact she's the only one of her species active in the novel, it might not be a critical mismatch.
The main character does end up in the walls of the ship, in basically service areas between the walls and the outer hull where there was access to wires and ship systems for maintenance. She got into those areas while spying - though spying on artifact smugglers rather than looking specifically for the alien, who is not known at this point - and there's a nice detailed bit where she's traveling along those areas to get from one part of the ship to another without being seen. She is not specifically chased in those walls, but she does get into them to avoid the smugglers and she is startled while in them (by the ship's cat).
It isn't a perfect match, of course, but the travel in the space next to the hull is pretty vividly described, and there is a dangerous alien disguised as a human, so it seems close enough to offer.