If you would like to close Mosaic but not shut down Trader Workstation, you can either toggle to Classic TWS or close down the Mosaic tab all together.
To toggle to Classic TWS, look for the Classic TWS button in the bottom left corner of TWS. Simply left click it to switch over to the Classic TWS layout.
If you wish to no longer use the Mosaic layout, right click on the Mosaic button in the bottom left corner of TWS and select Close Tab.
More information on the Mosaic interface can be found in TWS Users' Guide.