Seems odd if the bubble is sealed :? If it is really bad, and this won't be popular, but If it wasn't rare I might be tempted to try and lift the bubble on one side and then reseal it once the mold is cleaned off.
If it's just a tiny bit then a few days out in strong sun might clear it. But if it's a lot then you either leave it or try and get in there. One could argue that the mold will potentially damage the card further if left.
All depends on the card and the damage I guess. Got any photos?