Some brains on Fassbender. Its an actual scientific term:
I doubt he came up with it. It's probably just the descriptive term he's read from the script.
As was mentioned, 'Prometheus
' had some concept art for an 'ultramorph', which was basically just a derivative term of 'xenomorph' (which, in itself, was only ever intended to be generic, not specific to the species).
So, basically, they've come up with something which is meant to be a slight (yet recognisable) spin on the Alien design we all know about.
Which was what the old 'Engineers
' script was originally going to feature, don't forget...