Angel Feathers are the swept back looking (ducktail) feathers on female chicken's necks, just behind the muffs. The belief is they are recessive.
Several years ago I first found out about this when they started showing up in my LF silvers. My only advice is to breed from those without it, if you can. Sometimes a bird has so much going for it that you live with a fault like this, hoping to breed it out down the road. The Standard is silent on the subject, as far as I know, but Angel Feathers isn't a desired trait for Ameraucanas.
I'm used to seeing Angel Feathers, like muffs, on both sides of a bird, but I have one or more LF buff pullets with it only on one side. Here are 3 photos of a pullet. She has Angel Feathers on her right side, but not left.