That's an unbeatable recipe for hysteria and moral panic, and in medieval Europe, Jews, witches, lepers and Muslims were all regarded as incorrigible well-poisoners, which helped keep life interesting for them.
Hard to say what's going on between the lines of this little drama, but I love it for its Faulkner-meets-Poe-with-a-side-of-Erskine-Caldwell flavor. That Reverend Malpheus (great name!) thought he could pass himself off as a wayfaring stranger to Harlee Clay suggests that he had bought a bit too deeply into the stereotype of the gullible and childlike negro. Presumably Mr. Clay kept a straight face while entertaining his ofay guest. One wonders what the relationship was between the two, and how it came about that Clay had the local clout to effectively press his legal suit across the color line in post-Reconstruction Georgia. That would have required significant white patronage in local power structures, I should think.