"Swan" was one of Algarotti's nicknames. He was called the swan of Padua (and also some other Italian cities, because he moved around a lot) by both Voltaire and Fritz. You'll see him called "my swan" in the opening scene of "Pulvis et Umbra" for that reason.

I was taking "Principe" as Italian Algarotti making an allusion to the Anti-Machiavel, which Fritz originally wanted Algarotti to handle the publication of (before switching to Voltaire, who, at the time of this fic, is only one month away from publishing it, so it's fresh on everyone's minds), but I admit I'm not 100% sure on this one either. Will let our wonderful author clarify. :)