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

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.

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.