It is possible - only while a job is running - to access the Heritrix user interface on the harvester machine.
Start a browser on the harvestermachine and use the port specified, e.g. http://my.harvester.machine:8090. The port is defined by the setting settings.harvester.harvesting.heritrix.port.
Enter the administrator name e.g. "admin" and password e.g. "adminPassword" as set in the settings.harvester.harvesting.heritrix.adminName and in settings.harvester.harvesting.heritrix.adminPassword settings.

See in the installation manual how you change settings.

In the Heritrix GUI you can e.g. pause, stop or restart a job.