I'm wondering what setting is making this appear on emails below my HTML footer.
I don't recall changing anything - but is this option in the email studio, or is it in the Account Settings?
This is a can spam requirement and is added by default to every email. You can modify the message in the admin section of email studio. If you want to disable it, you can contact support or use an Ampscript hack. Note that it is important that subscriber be able to manage their preferences via subscription centre lest you find yourself In trouble with regulators