It's a cat. It got curious about the inner workings of a satellite and stayed there past liftoff to investigate. And now it's acting silly in front of the camera in the hope that the servant monkeys back on the ground can figure out how to send it a zero-G litterbox and a bag of Cat Chow.

(/s, of course.)

Article has your answer:

To test the potential of optical systems to revolutionize deep space communication, NASA incorporated the Deep Space Optical Communications (DSOC) experiment into Psyche, a mission to study a distant metal asteroid up close