First, I would say that reducibility is a property of statements. In the sense I use it:

The statement "14+14=28" is reducible to aether.

The statement "I have 28 apples" is reducible to phyisics.

The statement "There are 28 fundamental rules that one must obey to lead a just life" is reducible to ethics.

Moral statements are irreducible to physics in the sense that "P is red" is irreducible to physics - for any particular physical "P", it is reducible. The logical properties of P-statements, like "P is red or P is not red" are given as a set of purely logical statements - that's their analogue of the ought-function. If P-statements had some useful role in producing behavior, they would have a corresponding meaning.

Random, probably unnecessary math:

A reducible-class is a subalgebra of the Boolean algebra of statements, closed under logical equivalence. The statements reducible to aether are those in the reducible-class generated by True and False. The statements reducible to physics are those in the reducible-class generated by "The world is in exactly state X". The statements reducible to morality are those in the reducible-class generated by "Exactly set-of-actions Y are forbidden and set-of-actions Z are obligatory".