You mean the USB5V on the FTDI header? That’s only used if you’re supplying power to the board via an FTDI adapter when programming. You should definitely be using the SW5 pin for powering a sensor and not using the FTDI header.
What does your resistor divider network look like? What values, and how do you have them connected in relation to the sensor, and the power and ground pins of the Mayfly? Is the sensor properly connected to a Ground pin on the Mayfly as well?