«وَقَدْ أَقْمَرَ لَيْلُهُ الدَّجَوْجِيُّ»
His pitch-dark night had become bright with moonlight.
Al-layl al-dajawjī means a very dark night. Metaphorically it denotes once-black hair; aqmara suggests its new whiteness, as though moonlight had spread through the dark.
The narrative does not simply announce that he has grey hair. It dramatizes the change through darkness and light. That altered appearance explains why the narrator did not recognize a man whose language nevertheless seemed strangely familiar.