the default Fly Postgres server, last time I used it, doesn’t have SSL enabled for connections, so maybe a default has changed with hex package updates. Your error about SSL implies your trying to connect over SSL when the server isn’t configured for it.
In my config somewhere I would have something like this for a Fly DB:
config :spandex_app, SpandexApp.Repo,
socket_options: [:inet6],
ssl: false