Is "good", indefinable? Non-naturalism is a form of what's known as 'objectivist' metaethical theory. 'Objective' in this context means 'exists as something other than subjective states'. Moore positively rejects the idea that morality could be made of our own - or someone else's - subjective states, for that would be to reduce morality to something else.
And Moore himself was a realist. A 'realist' about morality is someone who thinks morality exists. That is, moral objects and relations are real. — Bartricks
But, see the quote by Banno above. It clearly states in the SEP entry that:
Moore’s non-naturalism comprised two main theses. One was the realist thesis that moral and more generally normative judgements – like many of his contemporaries, Moore did not distinguish the two – are true or false objectively, or independently of any beliefs or attitudes we may have. The other was the autonomy-of-ethics thesis that moral judgements are sui generis, neither reducible to nor derivable from non-moral, for example scientific or metaphysical, judgements; they express a distinctive kind of objective truth. Closely connected to his non-naturalism was the epistemological view that our knowledge of moral truths is intuitive, in the sense that it is not arrived at by inference from non-moral truths but rests on our recognizing certain moral propositions as self-evident, by a kind of direct or immediate insight. — SEP
To say that something in inherently intuitive (such as morality in Moore's case) seems to indicate that what moral claims represent are at least very subjective states, that are commensurably agreed upon. Do you think that's something correct to state?