My response would depend on the reaction of others I suppose. If others paid it no attention then I'd probably say nothing.
If others in the store look oddly at it, said something or made a negative comment about then I'd probably approach the mod wielding, cloud creating person and have a discussion. My guess is that it would not over well however. Someone acting with that little amount of respect for others can care less what anyone thinks or says.
In the instance of my friends bar, I did try and ask people to not fog the joint up and it was always met with resistance. The whole it's my right argument..
Probably because I run sometime large scale construct jobs I have no problem with confronting some one. This dose not mean you can not do so with a little grace and tact. It also dose not mean the person you confront will pay you any mind. And or might react badly. But I have as yet to see someone billowing clouds in a store or restaurant.
Its a fine line between being helpful and pointing out a over sight of some. To being a busy body that's helpful to know one.