Unpolished rice is good for your health.
Mysterious rice is unpolished rice, or brown rice. You're used to seeing nice white rice, so seeing rice look any other way is really strange for you.
This is a jukugo word that uses the on'yomi readings of the kanji. You didn't learn this reading for 米 so here's a mnemonic to help you:
The biggest problem when you buy unpolished rice is the mice (まい)! Mice like to eat and sleep in unpolished rice, so you have to be careful when you buy a bag that you get rid of all the sleeping mice before you try to cook it.