I would suspect that the ESC is not powering the motor.
I have no experience with ESC but looking at the connections, it looks like you have the battery pack connected via heavy wire and the red wire connected to the Arduino. The black wire is connected to the green jumper that is not connected to the Arduino. It looks like you have no Gnd connection to the Arduino.
You have a brushless motor there. Many times per revolution, the ESC will energise each coil in a positive direction and a negative direction. The average is zero. You could use the AC scale on the multimeter to get an idea of the voltage, but it won't be very precise.
You would need an oscilloscope to see the waveform and perform calculations on the waveform to get a real value that the motor is seeing.
Brushless motors with 3 wires are basically 3-phase AC devices (roughly speaking),
driven by a 3-phase bridge that may have PWM active on high-side or low-side drivers
or both. The tool to look at the output is an oscilloscope, time-averaged voltages will
not be informative.