You need to find a pokemon with a 3 stage evolution and evolve it from its second evolution into its third. The classic example here would be Bulbasaur -> Ivysaur -> Venusaur. If you have an Ivysaur and evolve it into a venusaur, this will count as evolving an evolved grass type Pokemon.
Here is a list of all pokemon with 3 stage evolutions. http://pokemon.wikia.com/wiki/List_of_Pok%C3%A9mon_by_three-stage_evolution . Having looked over it, here are the grass types that should work for this quest. There are a couple more, but you get the drift.
Ivysaur -> Venusaur
Gloom -> Vileplume
Weepinbell -> Victreebel
Bayleef -> Meganium
Skiploom -> Jumpluff
Grovyle -> Sceptile
Lombre -> Ludicolo
Considering how common Hoppip is, I'd say the best bet for this is to evolve Skiploom into Jumpluff as you should be able to find the candy fairly easily.
