The translation for Hunmatsu Ryokucha and Maccha are actually the same, green tea powder.
However, the ingredient for Maccha is a special kind of tea called "Tencha," in Japanese, and has unique "Umami."
"Tencha" is grown in the shade in order to prevent the final product from developing certain astringent bitterness, called "Shibumi," in Japanese. There are many kinds of bitterness. i.e. For Maccha, it is not necessary true that bitterness is bad. The 1st grade maccha is the most expensive and the least bitter; so it is good for drinking without adding sugar, at tea ceremony. The 2nd, 3rd, and 4th grade maccha is sufficient for cooking/baking/dessert-making/Maccha-Latte-Drink-Making.
The process of producing REAL Maccha is much more complicated and painstaking than Hunmatsu-Ryokucha.
Hunmatsu-Ryokucha is ground regular Sencha Green Tea that are casually served all over Japan in regular household. (Sorry, I am struggling with English.) But one must understand that the kind of tea plants, the way they grow the tea plants, the way they process the tea leaves, are all different, not only in Japan, but also in any countries.
(If you can read Chinese Characters...) Hunmatsu-Ryokucha consists of 4 Chinese Characters, Hunmatsu means powder, Ryokucha means green tea; however, Maccha sonsists of two Chinese Characters. Maccha means special green tea powder intended for tea ceremony. And we, Japanese, don't drink Maccha regularly. Most Japanese don't know a lot about tea ceremony either. (As most Japanese are not martial artists....)
In the United States, Asian Grocery Store sells both Hunmatu Ryokucha and Maccha. The real Maccha is usually in a nice tiny tin can, and the amount included is about 28g (about 1 oz),
The advantage of Hunmatsu-Ryokucha over Maccha is that the former has higher nutritional value, which I don't want to discuss in this site. But the cancer preventing effect in this cheap powder is higher than more expensive Maccha.
The disadvantage of Hunmatsu-Ryokucha over Maccha is of course, "Shibumi" (the closest direct translation is astringency.) That's why you may feel it is more bitter than Maccha. The powder is probably coarser than the Maccha.
You may use Hunmatsu-Ryokucha for baking and dessert making, in the same way as you use Maccha. But the flavor and taste are definitely different.
I have not tried Starbucks Maccha Latte. But I assume that they use the low grade Maccha powder since, amongst real Maccha, the low grade Maccha is less bitter than the premium quality Maccha for tea ceremony, I heard.
To conclude, if your purpose is to enjoy REAL Maccha flavor, you should not purchase this product.
But if your purpose is simply for your health, this product is better than Maccha. However, you should research availability and cost comparisons of Hunmatsu-Ryokucha products by actually going to the Asian Grocery Stores. You may get better
Also, Hunmatsu-Ryokucha is popular in Japan, too. Thus not all green tea powder imported from Japan is "Maccha."
If you want to buy the quality Maccha, you should find the Japanese Grocery Store (owened by Japanese) and ask specifically "Maccha" for tea ceremony. And if you are not satisfied, sign up for tea ceremony class taught by a tea ceremony master.
For the seller, one must not include "Matcha" or "Maccha" in the descriptions or titles, if the product was just "Hunmatsu-Ryokucha." As I wrote earlier, these two products share the same English Translation. So you can say, this is green tea powder, grounded green tea, powdered green tea. However, you must not even hint this is the same green tea powder that can be used for the formal Japanese tea ceremony. Simply, if you call this "Maccha" or "Matcha," you are lying. And you can be actually sued, and you will lose. Not only this seller, but I find many sellers are falsely including "Matccha" or "Matcha" for non-MATCHA GREEN TEA POWDER. So I believe this is just the innocent mistake. I encourage the sellers to correct their mistakes, though.