Is Nattou safe to eat when it has white dots on it? I bought a couple packs of Nattou (fermented soybeans) and stored it in my refrigerator. 

They come sealed in plastic, so I opened the plastic and only ate about half of one container. I wrapped it back up in plastic and stored it in the fridge again, and the next day I checked the Nattou and it was covered in tiny white dots, which look kind of foreboding. 

It did not look like mold, though, since the white dots were uniformly the same size and spread out evenly. In my experience fungi usually grows at random, and in different sizes, and usually larger than the tiny white dots that I see on the nattou.

What are those dots and is it still edible? 

I threw it away without trying it to be on the safe side, but I did notice this experiment is repeatable (I got white dots on the next package by doing the same thing the next day).

I think it might have something to do with storing it in the fridge, since I have stored Nattou in the freezer before without the white dot results.
Yes, those spots are normal, they form as the nattou ages. They are amino acid crystals, and they are perfectly safe. Here's a picture.



They're a bit crunchy, which you may or may not like. If you don't like the dots, get young nattou and consume it before the crystals form. If you do like them, get more mature nattou. With younger nattou, you should be able to store it opened in the fridge for a few days before the crystals start to form. As you have experienced though, once they start, they grow quickly.