I've had similar issues on my christmas moss from time to time. Although never to that extent. Looks like bubbles as you said. I tend to get it after a change in tank conditions. (Large waterchanges, moving the moss from one tank to another, sometimes if I make a change in floating plants, or trim/tear up the moss etc. ) Not sure what it is, it sometimes even feels a bit grainy if I rub it between my fingers, although it could just be the feel of the natural moss and my imagination.
I've never had any issues with it. Tends to go away over about a week and I never have had problems with the moss growing. I wouldn't be too concerned, just keep an eye on it. As long as the moss is green underneath its going fine.
It will take it a bit to get going, so don't worry if you don't see new growth right away. Just watch to make sure the moss itself doesn't turn brown or white. If its still around or gets worse in a week, then I would start to be concerned.