From the Alien Wiki:

"Acid Aliens are a type of Xenomorph subspecies. Judging by their name, they are known for spitting acid although they are certainly not the only caste known for performing this action. Their bodies are spindly and slim, while their head is significantly more elongated than many other castes. Their exoskeletal frame has a shiny look to it.

Xenomorphs that may have been Acid Aliens were seen spitting acid out of their 'maw' (AKA throat, tongue, or inner jaw). These potential Acid Aliens then latched onto a host, sucked blood and then injected their own acidic 'blood' into the host, killing it, mutating it into an Infectoid. It was also possibly spotted on BG-386."

Dial by Eric Schaen (continued below)

Pretty gruesome stuff, right? I think Eric Schaen nailed it with just the right amount of Blades/Claws/Fangs and acid spite. Sure it's 100 points, but this monster's got Indomitable and can tear spider-man to shreds with just a couple bites.

Add a touch of Shape Change and Toughness, and you've got one tough cookie!