I frequently hear that certain things should not be done because of Morris Ayin. People of all backgrounds seem to hold by him, yet I have been unable to find any sforim by Rav Morris. Who is he?
You've been hearing it incorrectly. The name is Moreh Zayin - the teacher of armaments. This is a metaphor for all those teachers who tell you not to do things because someone else may misunderstand you. These teachers stand with "virtual weapons" and give you a guilty look, so that you don't do what's forbidden.
Moreh Zayin come in different forms and they aren't necessarily school teachers. A parent who gives you a guilty look is also a Moreh zayin.