When I was traveling around Asia, I often ate Mixed Congee, though I never ate it in Japan (it may not even exist there).
The Mixed Congee I just ate (created in Taiwan by Taisun) is made of water, glutinous rice, oats, green lentils, peas, peanuts, cereals (whatever that means), red beans, longans (don't know what those are), and sugar. It's hardly sweet, but it's sweet enough to make you feel like you're not eating anything too boringly nutritious. But it's healthy enough to turn a lot of Americans off.
In fact, I was the only non-Chinese person at the Chinese store in Chinatown, so a lot of people either looked at me or moved away. I've gone to other Asian stores--Japanese, Korean, Vietnamese--and even though I was the only non-Asian in those places, they didn't look at me amazed, like "Is she in the right place? Oh my gosh--maybe she's not! Should we tell her? No, let's just look at her."
So now when I tell non-Asians my favorite snack is Mixed Congee, I can offer them a can for them to try. If they dare.