What about alien "ought"s?

There are no alien oughts, though there are alien desires and alien would-wants. They don't see morality differently from us; the criterion by which they choose is simply not that which we name morality.

There's a human morality in about the same sense as there's a human height.

This is a wonderful epigram, though it might be too optimistic. The far more pessimistic version would be "There's a human morality in about the same sense as there's a human language." (This is what Greene seems to believe and it's a dispute of fact.)