How to Stop Using an iCloud Password to Unlock a Mac
When a user sets up a new Mac, there is a helpful option to use an iCloud ID and Apple ID to login and unlock the Mac. Users can also choose at any time to have their iCloud ID as their login for OS X too. While this is undoubtedly a helpful feature and it can keep things simpler, requiring only a single password to remember, and making it easier to recover and reset a lost password on their computer, it can also have disadvantages for some situations. Additionally, some users prefer to use different passwords for different purposes for security reasons.
Whatever the reason, if you opted to use an iCloud password to login to a Mac when setting up OS X, you can later choose to disassociate the iCloud login and use a separate unique local login password on that Mac again.