Plutonium Oxide is part of the MOX fuel which was located in Fukushima Unit 3. It is also formed from the exposure of Plutonium to moist air. One direct method of formation would be the rapid conversion (2 day half life) of Neptunium 239 into Plutonium 239. Large amounts of Neptunium 239 have been reported in Japan, and the potential signature of Neptunium 239 has been detected in the Saint Louis radioactive rainfall of September 14th.
According to Los Alamos National Labs, Plutonium Oxide:
"plutonium dioxide is normally olive green, samples can be various colors. It is generally believed that the color is a function of chemical purity, stoichiometry, particle size, and method of preparation, although the color resulting from a given preparation method is not always reproducible. " .
Typically the color is reported to be "yellow-brown". Notice that the color and consistency of the substance from North Kanto Japan; it does bare a striking resemblance to the photos of known samples of Plutonium Dioxide provided by Los Alamos National Labs.
North Kanto, Japan: Typhoon rain fallout.
Click here to see the photo of the North Kanto Sample (the site does not allow hot linking)
Los Alamos National Lab Photos of Plutonium Dioxide