New file has an invisible, untouchable frame in all master pages

Hi all, I have looked through similar posts but none seem to cover this. I started a new file, but there is an odd frame on the master pages which cannot be selected, deleted, etc. See the attached screenshot; the blue frame is good, it's the pink frame I need to delete. I don't know how it got there, no clue why it's there, but I don't want to hand a client the file at the end with this messiness. Help please? Thanks in advance. 🙂