After securing application-to-database and replication traffic, you should also do the same for backup traffic.
If you use Percona XtraBackup with streaming than you should use SSH to send your backup to a secure location. The same is true for MySQL Enterprise Backup. Also both have options to encrypt the backup itself. If you send your backup to a cloud service this is something you should really do, especially if it is not sent via SSH or HTTPS.
And mysqldump and mysqlbinlog both support SSL. And you could use GnuPG, OpenSSL, WinZIP or any other tool to encrypt it.
You could try to force the client to send credentials elsewhere. This can be done if you can control the parameters to the mysql client. It reads the config from /etc/my.cnf, ~/.my.cnf and ~/.mylogin.conf but if you for example specify a login-path and a hostname.. it connects to that host, but with the password and username from the loginpath from the encrypted ~/.mylogin.cnf file.
You could use
--enable-cleartext-plugin to make it even easier to get to the stored password. Note that if you have direct access to the ~/.mylogin.cnf file that there are options to decrypt it.
See Bug #74545: mysql allows to override login-path for details.
MySQL Cluster (NDB)
Make sure your machines use a private network (VLAN) which can only be accessed from cluster nodes. Your API nodes should be in this network and have a public interface where mysqld listens. Another option might be to use a firewall device or host based firewalls. Just make sure you are aware or the risks.
As usual thers is extensive documentation about this: MySQL Cluster Security and Networking Issues from the MySQL Reference Manual.
And use proper security for iSCSI, NFS, FCP or any other kind of network storage you might be using. I've seen setups where iSCSI and/or NFS were publicly available and even with data-at-rest encryption this is not really safe, especially if read-write access is available.
In both MySQL 5.6 and MySQL 5.7 Oracle improved the SSL/TLS support a lot. There are more improvements needed as a lot has changed in how SSL over the past 10 years. Assumptions made years ago are no longer true.
And also the creators of YaSSL have been busy: wolfSSL/mysql-patch on github