Truth is nearly all of the corals labeled as being from Japan actually are not. It's a bad marketing term used on very colorful (sometimes not even colorful) corals to sell for more $$$. Japan has some pretty strict rules on taking corals out of their country.
The marketing strategy started because collectors from around the world select their most colorful and rare specimens and ship them to mainly Japan and Europe. US markets don't pay enough.
After seeing pics of colorful corals in tanks in Japan, people began labeling similarly-colored corals Japanese. HTH