Hall Sensor NJK-5002-C Configuraion
The output for that sensor is an open collector type. This means it is like a switch that connects the signal wire to ground (0V) when activated, and leaves it open circuit when not active. To observe this changing you need to connect a pull-up resistor (1K-10K should be fine) between the signal wire and the power supply. It should then work correctly.

The Chinese data sheet is very confusing, but I'm am 95% sure it is an inductive proximity sensor not a hall effect. These work by generating an alternating magnetic field at the blue end of the sensor. When a piece of metal (any metal, not just iron) is brought near this end, the magnetic field is affected and the sensor detects this and activates the output.