What the thing on Syrax's neck?

I'm late to the party and started watching yesterday. One thing that caught my eye, and I don't know why really, was this "spot" on Syrax's neck (season 1, episode 1). It's been a while since I watched GoT but I don't have any memory of seeing it on the 3 dragons there (could be because Syrax is lighter in color).

But, does anyone know what it is? I don't mind spoilers if that's needed to explain.