A long time ago before cats indoors was common

Oh wow. That’s pretty smart, but god all those (half) dead animals in the house. Our cats bring them in for entertainment..

In the films and everything, yes, it is just the blood that is acidic. That "acidic saliva" comment is a reference to The Devil Wears Prada song "Alien"

About 1:00 in this music video: https://www.youtube.com/watch?v=EiYMS1z8Lrw

EDIT: Lyrics

An extraneous existence
An earth-ending challenger
Encroaches upon humanity
Destruction, destruction they’re seeking destruction
Each ten feet tall without remorse
Crawling along the walls
I see the eyes, I see the teeth, see the monster coming through the window
Closing, the doors are closing
I see the eyes, I see the teeth, see the monster
One cannot fathom such a phantom
The demon’s mouth hisses with acidic saliva
Acidic saliva

Crawling along the walls
I see the eyes, I see the teeth, see the monster coming through the window
Closing, the doors are closing
I see the eyes, I see the teeth, see the monster coming through the window
Game over, we’re done for
Crawling along the walls
Claws extended
No hope to persevere
Closing, the doors are closing
We’ve been surrounded
There’s no way out
Game over, we are done for

EDIT 2: Okay, apparently "acidic saliva" is canon in Alien. I thought that was entirely just from that song (which is just largely inspired by Alien, not supposed to be based on it), but I see that some aliens did have that ability in the films and such.

The Drone and Runner castes have been shown to possess the ability to spit their blood at targets, although the quantity is small and consequently this means of attack is normally used to wound and disable targets rather than kill them. The spitting ability is used more aggressively by Praetorians, Snakes Rangers, and Spitters, which are capable of spitting lethal quantities of their blood significant distances, giving them a dangerous ranged attack other Xenomorphs do not possess.

https://avp.fandom.com/wiki/Xenomorph_blood