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I think its the ROM that's causing the problem. Most Pokémon ROMs show a white screen when they do not work.
I've experienced it, too, and what I did is downloaded another Emerald ROM. (and it worked for me)
But before doing that, try tweaking the options.
I think changing the save type or anything like that would work (though I'm not sure).
This kind of questions belong in the Emulation board down the main index. But yeah, most likely you don't have a working ROM: Try another, I have never had that problem.