Have the Colonial Marines met the Aliens before?

In the movie Aliens (1986), the following exchange occurs as the marines are on route to the stricken colony:

Hudson: Is this gonna be a standup fight, sir, or another bughunt?

Gorman: All we know is that there's still no contact with the colony, and that a xenomorph may be involved.

Frost: Excuse me sir, a-a what?

Gorman: A xenomorph.

Hicks: It's a bughunt.

I know that xenomorph is the name given to the Alien species in the franchise, which would mean that at the very least Gorman knows about them. Yet as the subsequent events show, the marines have no idea what they are in for, and are taken by complete surprise.

So what are the xenomorphs Gorman is talking about? And what "bugs" are they talking about hunting?

Answer

No, or they'd have taken off and nuked the whole site from orbit as soon as it became clear what they were dealing with...

Gorman uses 'xenomorph' because it means he can show off the fact that he knows long words in Greek, it's another instance of him believing that his Staff College education puts him above the grunts despite his lack of field experience.

As to the wider point, the whole conversation implies that there are other alien species that the Marines have come across (and also the mention of 'Arcturian poon-tang' whilst the Marines are eating breakfast) and that said species are at least hostile to humans if not actively belligerent. The Marines clearly place anything non-human (or humanoid, I suppose I have to exclude the likes of Bishop here) under the category of 'bugs' - the Bug Stomper artwork on the side of the dropship cockpit reinforces this.

The out-of-print RPG detailed other alien species that humans had come across, as I recall they were largely benign but there was at least one other species that caused problems for humans, I suspect this was an attempt to a) make the game not totally about Aliens, and b) give some substance to the referenced conversation.