During Nightmare Nights Icarus takes his job seriously; covering his skin in paint to make him look like he’s more skeleton than living batpony - at least in the dark.
So when the colts and fillies come to, “bring an offering to that old nag”, he makes sure they know Nightmare Moon doesn’t NEED to threaten them; She’s protecting them from her hungry guard.
“I hope you have a little something extra for him, too. After all, just LOOK at him! He’s practically Skin And Bones as he is.”