This looks like something out of an Arduino n-part “sensor kit”. It’s hard to tell because the quality of the image is not very good.
My guess is, this is an analog temperature sensor, like in this link, part number 18. You can also find an example sketch there.
"Arduino Pro Mini hooked up to the 5V power supply via the RAW and GND pins, but the supply is a regulated 5v wall supply. "
That is the problem. RAW pin expects more like 6V to 6.5V for the regulator to output 5V.
If you are supplying 5V, connect to the 5V, or VCC and GND pins.
If you are connecting to USB via an FTDI Basic or equivalent, and the 5V ...