Yeah he looked like he saw a ghost. I don't see what the problem is. The last Sharingan user died like 20 years ago, and then a new one jus appears before him. Also it should be noted that he looks like he was probably born after the Juubi was sealed. That means that it is likely that he has parents who have Sharingan as well (at least in Sasuke's mind because he doesn't know about the cloaked guy). He also looks nothing like an Uchiha.Adult Sasuke has fought numerous battles against the likes of Kaguya, Madara and the juubi. And not one panel has actually shown a sign of disbelief. Yet against a teenage kid with 3 tomoe Sharingan he basically was like he saw a ghost and eventually run to inform Naruto.