a) Any person who has an extreme desire to only associate with Asian or Asian descent people -- socially, sexually, etc.
b) a fetish for a certain thing, things/anything Asian.
b) "Man, he's/she's totally hooked on that (fill in term such as anime, japanamation, etc. here). Turned into a real rice monster."
The joke has no relation to either of the other two definitions, and most fans of Yoshiki telling the joke would be offended at either of them.
……. OH MY GOD!!!..... "I am so embarrassed"….. I've been doing a "special diet" and training, in order for me to play drums even harder….. Since then "THE RICE MONSTER" has been showing up in my dreams. Ha ha ha ha ha!!!"
- from a 2007 blog by Yoshiki on Myspace, and the origin of the term's usage