Based on the choices made since my first post I updated my design (the first version can be found on page 7). I reserve the right to repost this design as a final submission, (or label this post final submission, but that seems less appropriate).
Here it is:
Design choices:
A floating design was kind of silly looking back on it, especially now we know its ability is not levitate. The chosen stats have pretty high defenses, but kind of low health, so I figured it would fit if the pokémon was not especially bulky, but rather kind of hard to fully hit, with the flexible ghostly body and the tentacles to fend the attacks off. I did add sort of a wooden mask for a bit more of a bulky look, and because it fits both typings. the tentacles also function as a link to physical (grassy) moves like vine whip. power whip and constrict. furthermore it just felt like it needed at least some sort of appendages. I kept the ponytail sketch brush, because I like the look of it, it's yet another opportunity to show some plant/ectoplasm green. Furthermore I respectfully disagree with those saying sketch is not a (big) part of the design, it's the entire concept we're working from. A cameo appearance is the least I can do for it. We'll figure something out to not make it relearnable. I thought about giving it a mouth too, but for both a plant and a ghost that didn't seem to make too much sense.
On a side note I also removed the mist in the background, I figured that indeed wouldn't count as a plain white background. Also, this non floating variant seems decisively less misty.
Background story:
It is said that when the forest is in trouble the spirits of the forest will manifest themselves in the physical form of a [name], rising up to the challenge of their rivals, finding their weak spot and using their ghostly powers to learn a countering move.
Here it is:

Design choices:
A floating design was kind of silly looking back on it, especially now we know its ability is not levitate. The chosen stats have pretty high defenses, but kind of low health, so I figured it would fit if the pokémon was not especially bulky, but rather kind of hard to fully hit, with the flexible ghostly body and the tentacles to fend the attacks off. I did add sort of a wooden mask for a bit more of a bulky look, and because it fits both typings. the tentacles also function as a link to physical (grassy) moves like vine whip. power whip and constrict. furthermore it just felt like it needed at least some sort of appendages. I kept the ponytail sketch brush, because I like the look of it, it's yet another opportunity to show some plant/ectoplasm green. Furthermore I respectfully disagree with those saying sketch is not a (big) part of the design, it's the entire concept we're working from. A cameo appearance is the least I can do for it. We'll figure something out to not make it relearnable. I thought about giving it a mouth too, but for both a plant and a ghost that didn't seem to make too much sense.
On a side note I also removed the mist in the background, I figured that indeed wouldn't count as a plain white background. Also, this non floating variant seems decisively less misty.
Background story:
It is said that when the forest is in trouble the spirits of the forest will manifest themselves in the physical form of a [name], rising up to the challenge of their rivals, finding their weak spot and using their ghostly powers to learn a countering move.