This warning appears when your SSL certificate files aren’t properly protected. This happens when Really Simple Security has created an ssl/certs and ssl/keys directory in your websites root folder. The public certificate (*.crt) being available is not an issue, it’s the private key (*.key) file that needs to be protected.
To fix this issue, you can modify the file permissions to 644 for the .crt file and 600 for the .key file. You can also ask your hosting provider to do this for you.
Once this has been done, you can safely dismiss the notice.