I'm not aware of any such plug in device that flashes a light when current is being drawn, other than the type of device I posted. I know of a wi-fi immersion heater programmer that flashes a light when power is being drawn, but I suspect your pump may not draw enough to trigger it, and the device has to be wired into the cables, and be used to turn it on!
You can buy a plug in energy monitor, but that will have a screen displaying energy use, and you would have to look carefully to see if the figures are changing or stationary. Not exactly a simple warning device!
Could you fit a water level alarm so when it was down below a safe level for the pump, it would warn you? Better still - make that turn off the supply to the pump!