Click Sign Out in the bottom lefthand corner of the iCloud preferences window.If the popup occurs here, just click cancel.Usually, the reason this happens is that you changed your password or email address associated with your Apple ID recently (but it could be a variety of reasons). The real confusing part is that these popups often occur even when the user is not trying to access anything related to iCloud. This also occurs with Messages and FaceTime.
Occasionally, some users will report repeating popups asking for their iCloud Password. Simply doing this solved the problem for me, please let us know in the comments below if this doesn’t do the trick! iCloud Password
