So this is slowly been getting larger since I've moved into this apartment. I'm not sure what it is, but from doing a little searching it might be water damage? The wall is opposite the shower in the other room, so water might make sense.
lots like its been a mess like that for a while. it doesnt do something like that over night. probably all kinds of mold back there.the water problem needs to be fixed then it needs to be removed reboarded, taped, skimmed and painted. call your landlord to get on it.