I too was intrigued by this and did a quick google search. I came upon this which I think is a similar (if not the same) reaction as is occurring in OP. https

I too was intrigued by this and did a quick google search. I came upon this which I think is a similar (if not the same) reaction as is occurring in OP. https://thehappyscientist.com/content/simple-electrolysis https://thehappyscientist.com/content/simple-electrolysis And in the final paragraph states that, and I'm loosely plagiarizing here, the negative ions created in an electrolysis reaction are attracted to the positive/live terminal*, and in the presence of chlorine (as in salts found in the potato) the copper (wiring) react to form green copper chloride. Edit: not battery, but positive/live terminal