If it is your live sand, then, it is die-off. If it is not too bad, your tank, can adjust to the smaller cycle, without too much side effects. I had a similar thing happen with a part of a live rock, I put back in my smaller tank. I wanted to kill off some sponge, so I placed under hot tap water for a bit of time and during the week, that part of the rock, would smell and then, turn white. So it had some die-off, but since it was only a small part of my tank, I saw no other effects except for the smell.