Once a world has been added to a world server, it can be started and stopped at any time from the world list right-click menu.

To start a stopped world, right-click on the world and select Start World. The world will attempt to start. If the world encounters an error while trying to start, the reason will be indicated in the Error column of the world list.

To stop a running world, right-click on the world and select Stop World. Any users in the world will be unceremoniously disconnected, so it's a good idea to check the Users column first to see if there are users in the world before stopping it.