Hi Scott-
Thanks for the question, this should be pretty easy to fix. The most likely explanation for this is that the last time your Customizer was saved, your site’s URL hadn’t been updated to use https and was still on http.
To fix it, navigate to Customize > Background, switch to another background pattern, save, then switch back, and save once more. That should update the URL of your background image to use the same protocol that’s set in your Settings > General menu.