As a good practice, the connection between the Server and Database should always be over SSL, whether the network/connection is exposed to the extra world or not. In this case, there could an MITM attack by the person having access to the network. Just remember the principles like 'Segregation of Duties (SoD)', 'Need to Know' and 'Need to Have'. Unauthorized ...
Here is important to protect users' encryption keys on your application server in the way that provides post-compromise security:
What is post-compromise security?
Seamless key rotation:
Keys can be rotated while the app is running
Keys can be rotated both proactively or if there is a data breach
Invalidation of stolen data