Everyone has a face behind their mask, even Vader. What is Plo Koon's? Has anyone ever seen it, behind those eye-patches and breathing apparatus?
Assuming you're happy with an out-of-universe image, this is character actor Alan Ruscoe in his makeup, but without his trademark mask covering his mouth and eyes.
Interestingly, the Star Wars Databank article on Plo Koon used to contain an image with just his breather, however this appears to have now been removed.