Just like to remind everybody that very few Pokemon change type upon evolution (as in actually lose then gain a typing rather than just getting a second typing tacked on), and of those, most are evolutions of Eevee.
Honestly, I think that mono-Grass and Grass/Ghost are the only legitimate typings out there. It doesn't really make sense to have mono-Ghost since turning into a plant is a lot more difficult to invent reasons for than turning into a phantasm.
Also, I hardly think that Necturna screams Ghost/Grass - it's really about as overtly ghostly as Jellicent, Froslass, and Golurk. It lacks any really obvious "this is a ghost ok" design features aside from the red eyeballs everywhere and looking a bit creepy - certainly nothing along the lines of Chandelure or Gengar. It's far more easily identifiable as a Grass-type than a Ghost-type. In fact, if I had to guess, I'd say it was Dark-type, but that's irrelevant. Regardless, Necturna's design shouldn't really have to factor into the pre-evo typing - they're completely different things.
Right now I'm opposed to Grass/Ghost simply because Necturna's flavour is ridiculously individualised - as most CAP flavour is - in that it seems to be operating as a single entity, rather than having any relationship with or among its devolved forms. With Scratchet, initially we had Tomohawk as an ancient desert spirit or some such, which we then added to by making it the leader of a tribe of fluffy little ball things with claws. I wouldn't be opposed to Grass/Ghost if we could make the two distinct, in the same vein as Jellicent and Frillish (as in, Frillish grabs random stuff whereas Jellicent destroys ships and things, without any interplay between the two) but Ghost is a notoriously hard typing to do that with.
More to the point, mono-Grass is probably more likely to turn out a cutemon rather than some diseased shrub or something similar
come on we're all thinking it
Also Imanalt, you forgot Altaria.
EDIT: goddammit