Turning MSP430FR5969 off
You need to carefully think about two things: 'entering the off condition' and 'exiting from the off condition'. 

The MSP430 family is famous for its versatile operating modes. And the Low Power Modes LPM3.5 or LPM4.5 are your friends. In these modes all clocks are off, CPU is off, so the whole MCU is almost literally off. In LPM4.5 you have current consumption 20nA!

Before you enter the LPM3.5 or LPM4.5 you should finish everything: disable interrupts, save all the necessary and sensitive information from RAM to FRAM (as RAM is not retained), and decide about the way you will wake your MCU in future (if needed at all). Typically it's done by I/O interrupt (and it will be your only enabled interrupt).

For more info:


msp430fr5969 (SLAS704F) datasheet, especially chapter 6.3 "Operating Modes"
MSP430FR58xx, MSP430FR59xx, and MSP430FR6xx Family User's Guide (SLAU367N), especially 1.4 "Operating Modes", but better to read the whole chapter 1 as it contains critical system information.