Answer:
It's very possible that he saw evil as not necessitating an actual "being", but rather, an absence of a being. For instance, an action can be interpreted in many ways, and many different people ascribe different qualities/judgments to an action based on their preexisting moral views/philosophies. Emerson deemed these demarcations of actions into separate moral categories (exempli gratia: good and bad) as being too arbitrary, and chose rather to evaluate subjects of morality in one context, that is, the particular action's amount of good. If we view a moral system in which only good exists, (of course, in varying degrees) then evil can be apropriately defined as the lack of "good", and therefore the measurement of such becomes contingent on another property making it an inert, or unreal thing.
Of course, I could just be touting a load of BS. I'm not an expert in this field and I am not too acquainted with Emersonian philosophy, so a discerning eye could probably find many fallacies within my response.
I apologize for my sub-par writing skills.