shalom, and take us to your Seder...
I'm not Jewish, but IIRC the alien would have to be circumcised to take the Seder. That necessitates the alien having similar genitalia to humans, something unlikely. As such, inviting the alien to a Seder in which he cannot take part, would constitute a major "first contact" faux pas.
Just putting this out there in case any FReepers ever find themselves in this situation.