I have a ZFM-80US relay that, when it is manually turned on or off, generates these log entries, with the "Payload" indicating on "FF" or off "00". My question is how do I create an app that will respond to these changes?
You don't; you would use or write a driver that would respond to those Z-Wave commands and generate events as needed, then the app would subscribe to those events and do whatever it is configured to do in response. That is the foundation of how Hubitat's event-driven automation system works. See: Introduction to Automation.
But I suspect this is the wrong angle to look at this from, at least until you've ruled other things out. Just wanted to answer that specific question. I'd start with what @aaiyar is suggesting; there's likely a driver that can parse this information into events as required for your use, then you can use an app to respond to these changes -- quite likely all with built-in options (once you find the right dirver).
Thank you both. I have tried using both the generic Z wave relay driver and the built-in one specific for the ZFM - 80 US. but neither acts as expected.
This is one of those situations where I still just need a little more understanding I can takeoff on my own.
Problem solved: I'm using the "Generic Z-Wave Switch" driver.
These devices also required that the "Z-wave Poller" be installed, and all is now well.
Of course it now polls each fireplace controller every 10 seconds which seems more often than necessary, and it floods the logs. (Any idea how to control the polling interval?)
It's been years since I've looked at Z-Wave Poller, but if it doesn't provide options, you'd need something else--but it might be nice since I know it scales back if the device isn't heard from, which might keep your whole network happier if that ever happens. You normally won't see logs unless debug logging is enabled on the device or something is enabled in the app (though the traffic is there either way).
This device is old enough it might actually need polling, but if it's sending those Basic Reports on its own, it really shouldn't -- that looks like all the information it needs, just a matter of the driver parsing things correctly.
Thanks for reminding me about the 'debug log' switch - my log is much happier. They're working now, but clearly those old switches are on my 'replace' list.
I'd start with what 
