Does ground matcha need a hechsher? I know certain items, such as tea, when they are only one ingredient do not require a hechsher. Is ground matcha included in this?
Adding to alicht's post, I checked Wikipedia's explanation of "matcha":
Matcha is made from shade-grown tea leaves that also are used to make gyokuro. The preparation of matcha starts several weeks before harvest and may last up to 20 days, when the tea bushes are covered to prevent direct sunlight.[better source needed] This slows down growth, stimulates an increase in chlorophyll levels, turns the leaves a darker shade of green, and causes the production of amino acids, in particular theanine. Only the finest tea buds are hand-picked. After harvesting, if the leaves are rolled up before drying as in the production of sencha, the result will be gyokuro (jade dew) tea. If the leaves are laid out flat to dry, however, they will crumble somewhat and become known as tencha (碾茶). Then, tencha may be de-veined, de-stemmed, and stone-ground to the fine, bright green, talc-like powder known as matcha.
Grinding the leaves is a slow process, because the mill stones must not get too warm, lest the aroma of the leaves is altered. It may take up to one hour to grind 30 grams of matcha 
Based on this explanation, the production process involves grinding the tea leaves between millstones. Thus, if these millstones were used only for grinding tea and other similar natural products, and there are no other flavorings added to the tea (e.g., it's loose matcha powder), it seems that it would fit the rules mentioned in the OK article. Happy drinking.
According to the OK ("Consumer questions for the OK"), it would seem to depend on whether the tea is flavored or not:
Plain tea does not require a hechsher, since it does not have additives. However, any flavored tea does require a reliable hechsher, because the additional ingredients may not be kosher. Many flavorings used in flavored teas are made from dairy or non-kosher ingredients.
(see also: OU Kosher article "Tea and the OU Kosher Ingredient Approval Process")