I don’t get what the big deal is. When I want to clean the souls of the dead, I always spread tea-colored powder around a Japanese castle. Works like a charm 
Wish they would be more specific and there was more detailed information. Like what religion or sect or cult or something. What are they doing it for and why? Did they just randomly do it themselves or as part of something bigger, another organization or what?
Not so much info available from English sources.
Japan must have some good facial recognition systems at the airport.
Here’s an English-language article about it, from a Japanese newspaper:
https://mainichi.jp/english/articles/20180419/p2g/00m/0dm/103000c
Here are a couple of Chinese-language articles on it:
Not much information there in the news articles
I don’t recall Taiwanese “spreading powder” around in Taiwan for any reasons. Temples don’t do it. Don’t know about any Buddhist or Tao or folk religions doing it in Taiwan. So why are these Taiwanese women doing it in Japan???
As far as I can tell by using Google Translate, the Chinese-language articles seem to indicate that they may be suspected of doing similar acts involving other sites.
Edited to add:
No, apologies, I seem to have misread. It appears they’re suspected of spreading the powder at a number of sites in the castle: