"I eat green rice."
Translation:Ich esse grünen Reis.
According to dict.cc which I've heard many German professors recommend as a great internet source, der Reis refers to the grain, but das Reis refers to a graft or sprig of wood.
Occasionally in German you'll find words where a different gender confers a completely different meaning. I had a school teacher in Germany explain to me how even native German speakers will sometimes confuse das Schild (street sign) with der Schild (battle shield with your nation's crest on it).
"Das Reis" pl. "Die Reiser" is indeed a german word; it's a synonime of "Schössling" , "Trieb". Of course when one speaks about eating one think about "der Reis" but if one speaks about "eating green rice" then an educated german could even think about someone eating "Reis" with the meaning of "Schössling, Trieb"
The question is, how do you say "rice" in German? Singular or plural? I think in all the examples given so far, it's been singular. If they don't say it in a plural way, then doing it that way would be like saying "I eat green rices". Which could be valid, but the meaning is different.
"rice" is of course not countable. You can't say "one rice, "two rices", ...
But this is not the issue here. You used the definite form ("den Reis", "the rice") here where there is no article in the English sentence, so there should be an indefinite form in German as well ("Reis", "rice").