admin-ajax.php causing frontend problem

I am not an expert so please be gentle. Here’s the problem: I have my wp-admin directory password protected and since upgrading, I noticed that some visitors (especially mobile users) were accessing the file admin-ajax.php inside this directory. I wondered how that was possible so I logged out and tried accessing the frontend of my site and it asked for a password! Any thoughts on what to do? I’d like to keep the backend protected. Thank you so much.