I agree that it was out of line, but I don't find it personally offensive in the same way that calling MA "mulatta" is. I generally find that inappropriately accusing someone of being a racist is less offensive, objectively, than inappropriately being a racist (or throwing racist terms and ideas around for whatever reason).* In other words, I'm fairly immune to the charms of the race-card card. But I'm sorry you got your feelings hurt, and I do agree that it was inappropriate.
Just for the record, you're not defending a resident of the United States using the word mulatto knowingly/unironically and calling a peer "Mulatta," are you?
*Upon further reflection, the second use of the word "inappropriately" in this sentence is redundant, but I'll keep it for parallel-structure reasons.