We learn the glowing glasses are a reflection of the terror about to befall on him.
It’s up to interpretation, and that’s the beauty of Bly Manor.
But is he truly haunting her until she gets rid of the weight of his death or is he a hallucination?
Even before Dani signs on to become the new au pair to the Wingrave children, we catch sight of a man with glowing glasses in the mirror, haunting her through mirrors.