I installed catalina then downgraded to mojave using time machine backup. Now I have a second drive Macintosh HD data on my machine. Can I delete this?

I installed Catalina and decided I did not want it, then downgraded to Mojave using a Time Machine backup. Now I have a second volume on my desktop called "Macintosh HD - Data", which appears to be a Catalina created volume. How do I get rid of this if I no longer want Catalina and do not want Time Machine backing up this volume too ? (it appears Time Machine is backing up twice my original volume size).