The oscillating signal is 0.8v (pk to pk), quite a large swing.
The aquamist sensor generates a "sensor presence" voltage of 0.53V to alert the controller of its readiness to read flow. above this voltage, it is allocated for flow monitoring. The flow signal peaks at ~4.5V. Above this signal, it is a fault. So the sensor itself has inbuild failsafe to warn the aquamist controller.
Back to the plot. The BMS logging system appears to have picked up some noise and superimposed/modulated itself onto the aquamist's. The source is unknown but worth finding it out. It is definitely not from aquamist.
Jeff (aquamist US representative) has power up a similar age sensor to yours and sent me this captured image from his oscilloscope. this is what it looked like:
Superimpose (scaled) onto the BMS plot. it looks like this:
Where are those ghost oscillation came from? I am very interested to find out. Try disconnecting the green wire and make another plot.