Protect uart pin

The datasheet specifies a maximum rated current of 2mA in or out an input/inactive pin through the clamp diodes.

So you must ensure that the maximum current into an MSP pin is below 2mA. A series resistor >1.7kOhms will do.
Of course, this resistor will also limit the maximum I/O current when active. But usually, only a few µA are required to drive RX and TX. It will, however, affect signal rise and fall times, especially on higher line capacitance.

Keep in mind that this excess input current through the clamp diodes will go to VCC and effectively powering the MSP through the backdoor.

But there any hardware arrangement that I can to avoid current flow to Vcc pins when master is off & slave is on.

You can use a diode/pullup combination. So the slave can only pull the signal down, while it is pulled up to MSP VCC by a resistor. However, this slightly increase power consumption (by the pullup current when the slaves pulls the line down)
However, this only applies to inputs. For outputs it must be reversed: pulldown on the slave side of the diode, so the master can only pull the line up.
Don't forget the less steep signal edges into pullup/down direction (compared to an active pull) and teh diode voltage (a schottky diode might be required, so the low signal is detected).
In any, case, this is only for unidirectional lines (UART, SPI) and does not work for bi-directional lines such as I2C signals.