The AL2QXVFB runs as a started task executing the BPXBATCH program to the run a script in USS. If you stop or cancel the task from the console, the script running in USS is not terminated.
In order to stop the task and terminate the process running in USS, you will need to run the AL2QSVFB task. This task will execute the stopXvfb.sh script which will terminate the AL2QXVFB task and the script running in USS.