iCloud and iMessage accounts are different

iMessage is part of your iCloud services. Do you have more than one Apple ID? Did you sign into this iPhone with the Apple ID you use for your iCloud account? Are you having any issues signing into either iMessage or FaceTime using this same ID?

You would have to sign out of whichever service uses the AppleID you don’t want, and then sign back in with the AppleID you do wish to use. Every single Apple Service (iCloud, iMessage, FaceTime, the App, Mac, iTunes and iBook stores, these forums, etc) is tied to the AppleID you use to sign into that service.

On your iPad, tap on Settings > Apple ID > Sign Out. Then Settings > Apple ID > Sign In with your personal credentials.