It's a ghost!!!!!!
(Sorry, some Monday morning fun...)
Is there any protector film attached there? It looks like it's either a kind of scratch (more like sand blast) on the screen or it could be that it's actually lifting from the inside. I haven't seen the latter happen on a Nexus before, but it's not unheard of.
Is there a way you could take a closer picture focusing on the actual screen? The one posted focuses on the reflection, it's kinda hard to see the details of the blotch.