App Store login issue with prefilled unknown email address

I had an update notice this morning. I opened the app store and it showed that Twitter had an update.

On clicking Update I was presented with the log in box for the apple account. The dialog box was prefilled with an unknown icloud email address and was greyed out. I couldn't replace it with my own address. Has anyone come across this recently?

[Edited by Moderator]

Original Title: App Store Oddity