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 emaciated 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.