This corruption (green, square) forces you to swap one outskirts monster for one Arkham monster. No other effects, so until/unless the red corruptions start coming up and punishing you for having any at all, it's actually one of the few almost entirely positive corruptions.
The question came up in today's game: can you swap a monster from the sky with a non-flying monster in the outskirts? You could easily have no choice but to do that, if your monster control has been reasonable - only flying monsters refusing to land on the board, and no fliers in the outskirts. If so, what happens to the monster placed into the sky?
We played: you can swap them if you want, the monster can't move out of the sky but remains there to help reach the monster limit.
It's related to the old question from CotDP - what happens if you Deny a flying Hastur cultist - but there's the second question about whether it should be legal to move a non-flier to the sky in this sort of situation.