I'm looking for help locating a story read about 20 years ago. Aliens appear on earth, and for some reason, the human eye can't see the buildings they make.
At first, it's a novelty, but later, it becomes upsetting when they start building much larger buildings, and people can't do anything about it, because they're in turn protected by invisible walls around it.
At the end,
The aliens have constructed a large machine that, when turned on, forces everyone to just stop what they're doing, and line up in front of it, at which point they're all gathered up and killed in a "garbage disposal". While a small remaining number are taken and put in an alien zoo.