In a recent pamphlet from Sh'or Yashuv Yeshiva (5 Towns), I read that a shemittah etrog must either be eaten or left to rot until it is inedible.
Usually, when people see mold on food, they throw the entire package out, they assume that the food is spoiled. Some people, I know, brush or cut off the moldy part, and eat the rest of the food, as sometimes the food may still be edible.
Let's say that part of one's etrog has mold on the outside. The average person isn't going to test the etrog to see if it is still edible, because even a fresh etrog is quite bitter. Does he have to cut away a piece of the etrog to check if the inside still looks fresh and "edible", or can he assume that the entire etrog is inedible?