I know this thread is 4 days old now, but that's just a dragon pattern. It's not on all dragons. I think the dragon scales just stop where the white stops. Dragons described as full masks are the ones where the dragon scales cover their whole face and they don't have that stripe.
I have heard the stripe described as a "skunk stripe" but it's just something that a lot of dragons have, I'm not sure there's a specific name for it. If you search dragon scale on google you might find a lot of them. I personally LOVE the look of those stripes :)