Deleting auto-fill for usernames on facebook in safari

By mistake, I entered my username and password into the username box on facebook. Now my password is visible in the drop down box when I am logging into the site. I have tried deleting cookies, emptying cache, and adjusting my auto-fill preferences, and nothing has worked yet. I am a new mac user, and I recall on windows, I could highlight it and press "backspace" and it would erase.. can someone please tell me how to delete these automatic username suggestions.. Thank you