Hall effect sensor, water flow indicator alarm

The sensor board gives out a high signal (+5V) when the magnet triggers the hall sensor,

So the arduino will get a pulsed 5v signal?

I want the thingy to work pretty much as a switch, that reacts to the water flow

But with opposite function?

Read the pwm signal, and sound the alarm when the input pwm signal get below a certain set point?

But then the problem comes?

If the magnet stops at the location that the hall sensor is? Then it will trigger and be a constant a high/low signal?

High which the sensor puts out

Hall sensor module using M44 switch
With comparator regulating Hall signal
AO, real-time Hall signal output
DO, steadier Hall signal output
Comparator Output Current: 16mA