According to the article, the luminescence is caused by the presence of a large number of noctiluca, a plankton that glows when stimulated. The article says that the video was recorded on April 16 in Amakusa. A map in the video that’s embedded in the article shows the exact location:
https://www.ntv.co.jp/englishnews/articles/2021z0f0qg7be5yx5adz.html