I have come across this situation with one my clients’ sites.
It looks like the other site was once using the same droplet ip address like yours.
You will have to contact DigitalOcean and then ask them to remove the other site from their DNS - so that it will no more be linked to your ip address (because it now belongs to you)
Like they said, best way to tackle this would be by using virtual hosts file such that “only your domain name will be able to display the content of the website from your ip address” - you might need to take the help of a sys-admin for this though. Or if you’d like to do it yourself, you may ask check the serverfault.com site.
Sorry to repeat myself, but… setting up HTTPS is the easiest way to do that, as it requires no external server software – everything will be handled by the built-in Nginx server and Let’s Encrypt client
Did you try that already? If you got stuck, we’re here to help