There is a "Live Migration" portion and a "Background Migration" portion of the Migration process. Once the "Live Migration" is showing as complete in the "Migration Log", the appliance must be placed into "Operational Mode" to allow the "Background Migration" to run.
The "Stop Migration" button will be available if either the "Live Migration" or the "Background Migration" is running.
Please review page 55 of the Migration Guide.
To put the appliance in operational mode:
Once the background migration has finished you will see the Migration Log "All background tasks complete."