Got this from Exotic Orchard — had never heard of it before.
It’s apparently common also in Hawaii:
The Okinawan sweet potato is not related to the potato but is actually in the morning glory family. Native to the Americas, it was brought to Japan sometime between 1492 and 1605. The hardy plant grew well in Japan and quickly became popular in a variety of Japanese dishes. When it eventually made its way to the Hawaiian Islands, brought by the Polynesians, the crop flourished in the rich volcanic soil.
I cooked this a bit like the simmered taro root. Cut it into 1cm slices, added some water but not to cover, stirred in 1 Tbsp of shiro miso, and simmered until soft. Added a splash of mirin to finish. When cooked, the whole inside turns dark purple.
Sweeter, softer, and without the chalky texture of the taro root.
Just did a little more searching and found this Spruce Eats article with more info and some recipes:
Sweet potatoes are grown underground while yams grow on a vine above the ground, though yams do look like a tuber. The two are commonly confused and mislabeled at the market, so most people in the U.S. have never eaten an actual yam (native to Africa). There are purple varieties of both vegetables. The purple yam is the Filipino ube or ubi , and it is very rare to find it in the United States. It has a darker, rougher-looking skin that’s brown and reminiscent of tree bark. This vegetable is a major crop and food source in the Philippines. It is also made into a powder, which is then used in Filipino desserts that are unmistakable with their bright purple color.