The tool creates a temporary database from database migration scripts to perform database comparison faster. You will see an additional database named YourDatabaseName_ABCDEF.
Such database (but with different postfix) will be created for each developer working with the database in DB Version Control.
The temporary database will be removed when removing a project from Recent Projects list.
Is it safe to delete temporary database?
Yes, but it will be re-created automatically from migration scripts the next time checking for database changes.