How do I cite a guest lecturer that visited my class?
The 6th edition of the Publication Manual of the American Psychological Association is the definitive reference source for APA citation styles, however occasional unique examples aren't explicitly covered in which case it is recommended to refer to the APA's blog.
According to a reply post on December 19, 2012 from Timothy McAdoo, Trainer in APA Style, unpublished class notes (which would include unpublished class lectures or speakers) are to be treated as personal communications.
http://blog.apastyle.org/apastyle/2012/11/how-to-cite-a-class-in-apa-style.html