Interesting. It's a shame things like this have to rely on speculation now seeing as how they don't seem to want to explain this, probably because they don't know the answer themselves which is why they took this cheap way out of explaining anything. But then again, no one actually becomes a xenomorph when they're exposed to the goo.
In Prometheus, Halloway was turning into something but it wasn't a Xenomorph, Fifield turned into some zombie like maniac. The Xenomorph came from Dr. Shaw's black goo baby with Halloway but only after it infected the engineer that chased her.
I'm assuming the Xenomorphs from the spores on the engineer home planet in Covenant is from David's tampering. Also the worms that were exposed to the goo in Prometheus just became giant killer worms with enormous strength. Dunno, doesn't seem to be any sort of solid explanation you can nail on it. It's just quite puzzling that at the beginning its used to create life, but later on they're developing it as a weapon on that distant planet.
