Some fungi require dead organic matter for survival - necrotrophs. Many dont (e.g. yeast)
Saprophytic means: it eats dead plants. Necrotroph means: it kills a plant, and then eats the dead plant.
I have heard the term saprophytic many times, always related to fungi, but it's the first time i have heard the term necrotroph. My browser's spell correction underlines it with red like a word not spelled right. It is probably a new term.