The answer is, “it depends”. Generally an alternator is tied into your battery bank. Given the above graph, it seems probable that your solar charge controller is not isolating the solar connection from the battery connection, so that if the battery connection has higher voltage than the connection to the solar panel, that voltage feeds to the panel connection.
You can isolate that voltage using a diode. A diode only lets voltage flow “one way”, so it would let power from the solar panel into the charge controller, but prevent power from the charge controller flowing out to the solar panel. This would isolate the panel voltage and let the FloatHub see the panel voltage independent of other voltages supplied to the charge controller.
Having said that, your solar panels almost certainly already has a diode in place, but it would be be physically inside the panel somewhere, usually where the feed wires come from on the bottom of the panel. If you get a FloatHub sensing line on the other side of that diode, then you would be able to monitor panel voltage isolated from the controller. Accessing that connection may not be easy, however.