They're oxalis crassipes alba or more commonly known as white wood sorrel blooming at the garden of the Kujokan teahouse in Tokyo National Museum. The white color against the green leaves is so beautiful.
I visit the museum several times a year, but these days I go there to see netsuke on show indoors and plants and bugs in the garden rather than paintings and artefacts. Sadly for the museum, but luckily for visitors, the museum is quiet and relaxing with few people. If you have a chance to come to Tokyo after the pandemic, you should definitely go there. After enjoying art, you can also enjoy nice Japanese food and sake in Yanaka Ginza district!