If it
is meant to be G'raha, I think he does know us, he's just pretending not to. He doesn't want us to know it's him at this point. (If you choose to ask "who are you?" at the prompt, he tells you that's not important right now.)
Alternately, it's slippery double-purpose scripting that allows for either that interpretation OR the player genuinely not knowing him, without needing to include variable dialogue.
And the text slide at the end does say something about travelling across "time and space" - though it's worth noting that Minfilia also uses that phrase both times you meet her in the Aetherial Sea.
And as I
linked earlier, whether it is or isn't him, it is certainly a male Miqo'te NPC who matches G'raha's appearance perfectly.
But there's still room for uncertainty, and I'll be referring to him as Enigma until we know for sure whether it's him or a lookalike.