How would you do it?
One possibility is to give vague hints and see how Y reacts. Blurred streaks in the sky. Unexplained phenomena. What we see now once in a while, but I'm sure that's coincidence.
![Smile :)](./images/smilies/icon_e_smile.gif)
Another possibility -- land on their equivalent of the White House lawn? Assuming you know their language decently. That you make your intentions clear. That either no guard with a twitchy finger shoots your envoy, etc. Smiles and handshakes all around.
What happens to the stock market? Technology companies may find their work obsolete, but they are also best positioned to take advantage of alien science and technology. The market doesn't like uncertainty. And that's not just fat cats' money, that's retirement funds and money banks and business uses for, in part, jobs. "Aliens come in peace!" might be followed by "Breadlines Open Up."
National defense... If the aliens landed on the Y-equivalent of the White House lawn, would people in Y-Moscow celebrate? Medicine... If the aliens can cure everything, Ys live longer. If they can really cure *everything*, we may live much longer. Population will go up. If Y is handed cheap fusion power on a plate, that helps the impact of that population, but that's not true for every resource.
How do you as X first contact Y?