Purple column on the left??? Help!

Hi and tnxs for reading,

today column on the left appeared (not in my document but in the view of one of my document).

This happened only on my mac, while the same document on iPad or iPhone (it is on iCloud) is properly shown.

How can I go back to the standard view??

I am attaching a shot just to give an idea.

Tnxs, cheers.