Masterswarm

With a more macroscopic interpretation, the song's subject could be America, the land of immigrants. Particularly "come by sea / swarm like smoke in the dawn." Naturally, there is nothing anti-immigrant to this interpretation; the reference to the swarm is inclusive and in the past-tense (we were the swarm) perhaps offering a reproach to those who would discriminate against immigrants although they themselves are the descendants of immigrants. Furthermore, the swarm turns out to not be parasitic (as I noted above), but developmental: the final product of these larval swarms is a man "who speaks with perfect diction" and "acts with more conviction"... in other words, the swarm eventually improves upon the host.

So if the song is about immigration, the swarm represents the people who flock to America's harbors (I can see the Statue of Liberty welcoming them with the opening words: "come what may") and ultimately improve upon her being.

I also want to point out the wordplay in the line "in vestments of translucent alabaster." Yes, the skin ("vestments") of larvae is like translucent alabaster. But the line can also be read as "investments of translucent alabaster," which ties in nicely to the idea of the swarm developing into something better than the host.