It was Darla who cursed Benjamin Rash, with her mistletoe wand. The spectral residue was from Ben Journey, who (as he told Alexandra) arrived at the scene of both incidents, but too late to see who was responsible or what happened (or so he said...).

I'm not quite sure I understand your second question. Do you mean the Generous Ones won't know that Darla sacrificed herself? This may or may not be true. But there were some other hints.