Unfortunately, it is not possible to change your Milotic's Hidden Power; in fact, it isn't possible to change any Pokemon's Hidden Power under and circumstance.
The reason for this is how a Pokemon's Hidden Power type is determined: through IVs. Since IVs can never be changed, the Hidden Power type cannot be changed either. You can read a full article on how IVs determine its Hidden Power type by clicking here, but I will warn you that it is more complicated than you'd think.
The reason your Milotic has Hidden Power Dark is because its IVs meant that it got Hidden Power Dark (the article I linked above will explain that process).
So basically, the Milotic you have now will never have Hidden Power Grass. To get a Milotic with Hidden Power Grass, you'd have to capture another one that has IVs that will result in Hidden Power Grass.