Usually when you get "suckback," it's due to temperature differential, but even a change in weather, specifically the barometric pressure, can cause it. Had any storms in your area lately? It all boils down to lower pressure inside the carbouy than outside. There are a few things you can do to prevent it, like switching from a 3 piece airlock to an "S" shaped airlock, or you could use a blowoff tube.
Another thing you can do is to use alcohol (cheap vodka works well) or even Starsan in your airlocks instead of water. This won't prevent suckback, but it will prevent anything nasty getting into your cider. Now as for what to do with your current batch...
If there really is mold on the surface, I think you did good when you hit it with campden, but you might want to try racking from under the mold if you can. If you can remove the mold from the cider, or more accurately, remove the cider from the mold, you might be able to nip it in the bud as it were. You might want to hit it with campden after you rack, just to be on the safe side.
Hope this info helps. Regards, GF.