And who knows maybe it is his skin, but maybe his skin was magically imbued with some sort of ash or something, hence the crusty greenish appearance.
Kotal seems partially inspired by their inclusion of Kratos in the last game (War God variant says hello). For those not in the know, Kratos' skin is covered in ash via a curse. Maybe Kotals' skin is ashen green because of some curse or ritual performed on him.
So it could both be his skin and be covered in something. Being magically grafted to his skin would make it so it wouldn't rub off similar to the ash on Kratos.