Facing warning message while creating tar.gz file
As you have said, you are trying to create a hot image copy backup for cloning and recovery purposes, it's not the recommended way to take image copy backup. The file may contain fractured blocks-which have been changed during the copy command(tar in your case). The tar command has no intelligence to check whether the block has been changed or not during the archiving process resulting broken block known as split or fractured blocks included in the file.

Warning from tar command has spoken the same theme as I mentioned here. The file was being modified by Oracle processes, possibly doing read/write operations, as you said, you were trying to archive these files while the database is up and running. 

Instead, if you use RMAN it has the ability to know whether the block is fine or changed by comparing the header and tail of a data block.

Documentation: RMAN Backup Concept/Image Copy Backup