Maybe for the ferroseed guess "steal the greener side" could be a reference to steel type and grass type? Not my guess though...
I'd say hoppip.
Spinning these things around and around.
"These things" refer to its leaves, which spin in pokemon mystery dungeon.
They hardly ever touch the ground.
They, the the leaves, almost never touch the ground, and neither do the hoppips, because they are carried around by the wind.
I may be a pain in your side.
For I steal the greener side.
Many of its moves are centered around stealing HP - leech seed, absorb, giga drain, etc.
The first think I thought was Hoppip, but as someone else has already said that I'll say Jumpluff.
It spins (?) those cotton things, maybe, to stay airborne (I know Hoppip does that). It hardly touches the ground as it is part Flying. It can be a pain to deal with competitively because it has annoying moves like Encore and Sleep Powder, and it can steal the foe's HP with "Green" moves like Leech Seed and Giga Drain.
I think it's gastly because its gas floats all around its body, it will vary rarely touch the ground if at all, the gas is poisonous and will hurt you, and finally it will suck the life out of things including plants.