Spock had years of experience dealing with beings of other species. So when Jareth pulled away from him, now comfortable in using his own legs and keeping himself upright, Spock respected the man's physical space and dropped his arms to the side. Which was not to say that he wouldn't reach out another helping hand, should the man with the wild hair require it, but Spock knew very well the importance of keeping a distance where strangers were concerned. It was impossible to know the results of being too helpful.
"Your gratitude is not necessary."
Spock had provided a function suiting to the situation. The situation had passed. It was irrelevant whether the man appreciated his assistance or not.
"I am Spock, First Officer of the U.S.S. Enterprise, representing the Federation of Planets. You may have seen my ship prior to your isolation in the asylum."
The Enterprise, like many of the buildings in The City, had the tendency to move across The City's sky from place-to-place without warning. This made it difficult for Spock to return to the ship to continue planetary scans (well, scans anyway -- he had yet to determine if The City was a planet.)