Hey Molly! Yes, my dog has come between me and strangers a number of times. Fortunately, simply saying no to them will have them dismissing the stranger and getting back to what they were doing.
I think the reason why your dog started to growl was because it felt intimidated by your mom's tone of voice, which is good given that he's capable of determining the tone of threat and what not.
Back in the city we lived in, nobody dared entered our home because my mom had 5 big dogs which barked with big voices. Actually, they're just barking out of curiosity. Nonetheless, people in our neighborhood has perceived this as aggression.
