Reiger's approach was indeed noticed by Nightmare.
Naturally, it induced fear in him.
As a Lower Rank One, the demon closest to the Upper Ranks, there were very few things that could instill fear in Nightmare.
To him, ordinary Ghost Hunters were no different from regular people.
Even against the Pillars, Nightmare was fearless, even eager to kill more to gain merit.
For demons, the constant ghost-hunting Demon Slayer Corps were a thorn in their side, and the master had long since issued orders to kill the Pillars, so Nightmare had never feared them. Otherwise, he would not have concocted a plan to merge with the Infinite Train and consume both the Pillars and two hundred passengers in one go.