The MaxDB X Server is needed only for network communication. See Concepts of the Database System, Network Communication.
For local communication, that is, when the database instance and the database application (or database tool or the interface) are on the same computer, MaxDB uses shared memory. Two exceptions are the Event Dispatcher and the JDBC interface, which require the X Server even for local communication.
You are using neither the Event Dispatcher nor the JDBC interface.
Stop the X Server with the following command:
X Server, Stopping the X Server