A is the Power LED light. When this LED light is flashing blue, it is receiving power and on "Standby." When this LED light is solid blue, it is in "Clean" mode.
B is the actual CLEAN button you press to toggle between "Standby" and "Clean" modes.
After every cleaning cycle, Mirra will automatically be put in "Standby" mode (flashing blue light will appear). If you want to turn Mirra off during its cleaning cycle, press the CLEAN button, which will cause the blue light to go into flashing mode again.
When the cleaning cycle is finished and you've removed the Mirra from water, unplug the power supply, so you can do regular cleaning and maintenance of your robot.