Help — Am I being hacked?

I apologize for somewhat repeating this old question, but I haven’t found an answer so far ??

I setup my blog so that new user default role = Author.

This happened to me twice recently: I noticed a new user registering (without posting anything). Later on, whenever I myself post or edit something, the new/edited post (or page) is now granted this new username as an Author.

Is this some kind of hack? How can I fix that? Also: Is there a way for myself, as the Admin, to edit the author username of an existing post/page?

Please help…