There's a harshness to that 'idiot' that made Fay think Kurogane must be in quite a bit of discomfort. A tried stubborn idiot crossed his mind. As with a lot of the meaningless names Kurogane had called him in the past, Fay let this one slide, but then, there isn't a lot he wouldn't forgive this man for.
"Mmhm," Fay nodded his head and dusted his hands down his clothes before he picked up the vase again, and stood. "But not with these dusty hands. Come inside and we'll get Kuro-ron cleaned up."