It's acetalaldehyde. Those cidery/green apple flavors are produced in a few ways: One is just incomplete fermentation, another is oxidation of alcohol into acetic acid (most likely in your case), and the third is contamination by an acetic acid bacteria.
If I had to guess, i'd say the beer was oxidized when you racked it into the keg. Next time when you clean out your kegs, just use the Star San and a scrubby brush. Be very careful to slowly rack your beer using a slow flow racking cane that's completely submerged. An auto siphon works well to transfer your finished beers, too.
Beer so good, it's frightening.
2014: Seven Heavens Series One: Vilon, Mystic Melomel, Skeeter Pee.