Wikipedia claims that the word Xenomorph was indeed coined for ITCFOS's ad campaign (there's actually a whole entry for Xenomorph there, describing the creatures from the Alien film). I found the following posters using the term "Xenomorph", but both of them are for re-releases, one of them a double-bill with Creature from the Black Lagoon. Note near the bottom of the first one that there is an MPAA "G" rating; you can see it full-size at the Movie Goods website:


Interestingly enough, I also found out that it has two different French-language titles. The French poster calls the movie Le Meteore de la Nuit (Night Meteor), but the title on the Belgian poster is Inconnus d'une Autre Monde (Strangers from another World).