I picked up a few pieces from someone on this site last year. When I got it home I found it had a bunch of mojanos on it. The outside temperature during the next 24 hours was down into the 20's, so I decided to experiment with it - I pulled it out of the water, wrapped it in a plastic bag, and let it sit on my porch for a day. The next day, I took the frozen rock out of the bag and placed it directly back into my tank. Since this was a new setup, I wasn't worried about the water temp dropping. The next time I saw those mojanos, they were shriveled up, fell off the rock, and more or less disappeared. Never seen another one since.
I was hoping that the beneficial bacteria could survive the deep freeze.