This is a rare surname and it appears to have escaped the attention of the experts. Franco on it's own is a male given name meaning 'free'. But it could also just signify a freeman, as opposed to a serf tied to a particular master. Macaro looks like it could be a variant of another male given name, Macario. So - just speculating - it might have identified someone called Macario who was a freeman. Or it might be two forenames combined, as is the surname Marcantonio. Macario is from the Greek Makarios, meaning 'blessed'.