By typing in your public IP (to your broswer, I suppose?) you just reach the web interface of your nas. This is the default, normal behavior. You need to specify the FTP protocol in the URI if you want to hit the ftp server or use an ftp client which will automatically connect to the ftp port and not the web server by default.
If you are using your browser, try the following address:
ftp://YOUR_PUBLIC_IP_OR_DDNS_NAME