So it's an idiom, after all. Imho, πλάκωσε refers us to a "heavy" shadow, because of a big problem, maybe; very aptly translated as «Το πρόσωπό του σκοτείνιασε». :)In modern Greek we would rather say «Το πρόσωπό του σκοτείνιασε»
I don't consider «ένας ίσκιος πλάκωσε το πρόσωπό του» an idiom because -from what I know- it's not commonly used or included in dictionaries like "a shadow crossed his face" in English.So it's an idiom, after all.
So it's an idiom, after all. ...