Arya hasn't seen Sandor Clegane since she stole his money and left him for dead after Brienne of Tarth beat him within an inch of his life at the end of season four. Considering she's had his name crossed off her list for the past few years, her reaction to finding him alive and well in the forges is remarkably cool-headed. Arya's relationship with The Hound has always been a complicated one, and their encounter here is no exception, with The Hound displaying grudging admiration for Arya's ruthlessness, and Arya offering only the tiniest of eyebrow quirks in reply, giving away none of her inner thoughts. She could just as easily be plotting to poison him in his sleep as bury the hatchet; only time will tell which.
It's an intriguing little moment between two of the show's most fascinating characters, and while we have no idea whether these two will remain allies once the White Walker threat is past (assuming they both survive), it was fun to see them finally get to share the screen again.