Based on your description of a plant with green structures on tall brown stalks, it is likely that you are observing a type of grass or grass-like plant, possibly even a type of reed. The green structures you see are likely the leaves of the plant, as many grasses have tall, slender stalks (culms) with green, blade-like leaves emerging from them.
If the plant is particularly tall and has been observed in wetland areas, it could be a species of reed, such as common reed (Phragmites australis). Alternatively, it might also be a cultivar of ornamental grass, which often has a similar appearance.
If you could provide more details about the plant, such as its height, habitat, or whether it has any flowers or seed heads, that might help narrow it down further.