Ruptured air sac. It will go away. I've had it before, but never on a NN and I bet the nude neck makes it look worse than it is.
If it was bigger and continued to get bigger you would need to poke a hole to let air out or it will tear. It's usually caused by some type of trauma (tramped or something).
The air normally is absorbed back into the tissue. That's always how I've seen it happen. I've had 3 chicks over the last year like this. All were fine after a while.
Thanks Aoxa - I was soooo worried! Would it be best to separate the two turkens for a few days just to be safe. My other one looks like it might have a small one too... If it gets bigger, what do you suggest I use to let the air out?