"Holden's unpleasant dormmate, whose personal habits are dirty and whose room stinks. Holden suspects that Ackley does not brush his teeth and describes them as mossy. Cursed with acne, Ackley constantly picks at the sores. Ackley dislikes Stadlater, calling him a "son of a bitch." Holden finds Ackley disgusting but appears to feel sorry for him at the same time."

Holden's dorm neighbor, Robert Ackley, is the type of person who suffers from poor hygiene.  He doesn't brush his teeth properly, he is always clipping his nails and leaving his clipped nails in other peoples rooms.  He has terrible skin and is very nosey.  I think that Holden dislikes Ackley's nosey behavior the most, he is sympathetic to the guy, but he can't stand it when Ackley walks around his room and touches everything. 

"He started cleaning his fingernails with the end of a match.  He was always cleaning his fingernails.  It was funny in a way.  His teeth were always mossy-looking, and his ears were always dirty as hell, but he was always cleaning his fingernails.  I guess he thought that made him a very neat guy." (Salinger) 

Ackley tends to barge into Holden's room without being invited.

"Hi I said, but I didn't look up from my book.  With a guy like Ackley, if you looked up from your book you were a goner.  You were a goner anyway, but not as quick if you didn't look up right away.  He started walking around the room, very slow and all, the way he always did, picking up your personal stuff off your desk and chiffonier.  He always picked up your personal stuff and looked at it.  Boy, could he get on your nerves sometimes." (Salinger)