The face in the darkness is a floating head who appears in the episode "Night Light."
He is simply a floating random male human head shown in the darkness of the freezer.
Role in episode
He appears when Mr. Krabs asks SpongeBob to get some fresh Krabby Patties from the cooler. SpongeBob then imagines a head appearing inside it, frightening SpongeBob and causing him to scream and cling onto the kitchen ceiling.