The Collaborative International Dictionary of English v.0.48:

Mealy \Meal"y\ (m[=e]l"[y^]), a. [Compar. Mealier; superl.
   Mealiest.]
   1. Having the qualities of meal; resembling meal; soft, dry,
      and friable; easily reduced to a condition resembling
      meal; as, a mealy potato.
      [1913 Webster]

   2. Overspread with something that resembles meal; as, the
      mealy wings of an insect. --Shak.
      [1913 Webster] mealybug