I think there's an ambiguity between "realism" in the sense of "these statements I'm making about 'what's right' are answers to a well-formed question and have a truth value" and "the subject matter of moral discourse is a transcendent ineffable stuff floating out there which compels all agents to obey and which could make murder right by having a different state". Thinking that moral statements have a truth value is cognitivism, which sounds much less ambiguous to me, and that's why I prefer to talk about moral cognitivism rather than moral realism.

As a moral cognitivist, I would look at your diagram and disagree that the Baby-Eating Aliens and humans have different views of the same subject matter, rather, we and they are talking about a different subject matter and it is an error of the computer translation programs that the word comes out as "morality" in both cases. Morality is about how to save babies, not eat them, everyone knows that and they happen to be right. If we could get past difficulties of the translation, the babyeaters would agree with us about what is moral, we would agree with them about what is babyeating, and we would agree about the physical fact that we find different sorts of logical facts to be compelling.

I have a pending post-to-write on how, to the best of my knowledge, there are only two sorts of things that can make a proposition "true", namely physical events and logical implications, and of course mixtures of the two. I mention this because we have a legitimate epistemic preference for simpler hypotheses about the causes of physical events, but no such thing as an epistemic preference for "simpler axioms" when we are talking about logical facts. We may have an aesthetic preference for simpler axioms in math, but that is not the same thing. If there's no preference for simpler assumptions, that doesn't mean the issue is not a factual one, but it may suggest that we are dealing with logical facts rather than physical facts (statements which are made true by which conclusions follow from which premises, rather than the state of a causal event).

Added: Since I have a definite criterion for something being a "fact", I defend the notion of fact-ness against the charge of being a floating extra.