It does look like a snail shell. My first thought is maybe it's the remains of some sort of predatory snail. Here in Calfornia, snails with similar spiral shells have been introduced.
UCANR 2019- Home Page
ucanr.edu
But there are other species. It seems there's also one from the Southeast?
en.wikipedia.org
But if it truly has a barnacle on it, maybe it's just a sea shell that somehow got mixed in with the soil in your pot? Odd, but not impossible.