They really don't have any other words they've used for non-evil-alien-people besides "filth creature", do they.
"I want you all to stop killing filth creatures," he says.
"Please do not befoul us," says the spokes-alien. "Although the [same untranslated plural noun] will likely kill us anyway."
"[Untranslated adjective]! [Untranslated adjective]! We're [untranslated adjective]!"
"Shut up [profane intensifier]!" says another alien to the one who thinks they're all [untranslated adjective]. The adjective-wailer subsides.
"The end of the world!"
"Shut up [profane intensifier], [probably a name]!"
"Filth creatures would rather," (thank you spokesalien) "not be attacked."
"Shut up [profane intensifier], [name]!"
"Nothing else has stopped this filth creature, why would destroying the planet even help?" someone wonders.
"Tell that to the [same untranslated plural noun]!"
"You tell them!"
The spokesalien has nothing more to add at this time.
"[Untranslated] is the opposite of filth," explains the spokesalien. After a bit of digital hemming and hawing, the computer renders its next sentence as: "The purity-keepers maintain purity so that no one is contaminated by filth. This is the most frightening colony planet, because of the filth contagion, but until today there had not been an incident in many thousands of years!"
"[Untranslated sound]..." says the spokesalien, gesturing vaguely at Cam. It tries a few more phrasings, and the computer finally pieces together: "Grew wings. They had no wings, and then they grew them. Wings are a limb only filth creatures have. People have four limbs, but the filth creatures of this planet sometimes have six..."
Cam resists the temptation to wave his wings at them. "Why is it filthy to have wings?"
Cam does not have the word for 'circular argument'. He makes a butterfly. "Is this a filth creature, too?"
"It lived here when filth creatures did. It has wings." The butterfly bats them.
Did they just kill all the butterflies? He didn't get a really close look at the vegetation, maybe it's all alien stuff, maybe they killed the whole biosphere. He flicks the butterfly away. It flies.