The user puts a forest curse on the target. The target is now Grass type as well.
Forest's Curse adds the Grass type to the target, in addition to the Pokémon's original type(s).
If it is used against a dual-type Pokémon, that Pokémon will have three types at once. If the target already has an additional type added to it by Trick-or-Treat, that type is replaced with the Grass type. Forest's Curse will fail if the target is already Grass-type.