"Time Machine is backing up an encrypted disk to an unencrytped disk"

It’s a one time only informational message, ignore.

Just a warning(reminder) that while your internal drive is encrypted your external is not and is open to file theft if ever stolen. You can always have your backup drive encrypted.

FYI if an unencrypted TM backup disk becomes lost or stolen, it can be used to effectively create a duplicate of the Mac it's backing up without requiring that password. Keep that in mind for when the disk eventually fails and is no longer useful for TM.