Impostor

Impostors are a sapient species that has infiltrated multiple facilities of the Crewmate space program with the intention to kill everyone working in them. An Imposter perfectly mimics a Crewmate space suit, except when its mouth opens to use its tongue as an impaling weapon. Impostors have superior low light vision to crewmates. They also have technology to remotely sabotage insecure Crewmate devices.

Biology
While their true form is unknown, their disguised form copies the form of a Crewmate's spacesuit (assuming Crewmates ARE wearing spacesuits anyway). The only indication of their alien anatomy is their sharp teeth, mouth located at the stomach, and sharp tongue. Imposters appear to not require oxygen to breathe, and can survive in extreme conditions, possibly even in space.