this exact thing happened to me, only it was a 200 gal. box. the bag i was working on all of a sudden started dong this white flake thing too. opened up a new bag (last one in the box) and it did the same thing. call this lady at instant ocean, Rosemary hartman 1-800-822-100 ext.16115
She will set you up with a replacement bucket but she will want you to send in a small sample of your dry salt so they can test it.
i tested my bad water and found it low in DKH and calcium. I believe the problem was that the salt got moisture in it somehow and the water that you make will have the white snow....i think the calcium and the carbonate/bicarbonate are precipitating out.
i did the water change anyway (let the white snow settle to the bottom of bucket). buy a small bag of salt at the lfs to tide you over until the replacement arrives.