Mold or flowers of wine, either way, I think you can save it. I had something similar in a cyser once, it looked sort of dry & powdery on top & just floated on the surface of the liquid. I thought it was flowers of wine & used Jack Keller's treatment for it:
"Flowers of Wine: Small flecks or blooms of white powder or film may appear on the surface of the wine. If left unchecked, they grow to cover the entire surface and can grow quite thick. They are caused by spoilage yeasts and/or mycoderma bacteria, and if not caught at first appearance will certainly spoil the wine. If caused by yeast, they consume alcohol and give off carbon dioxide gas. They eventually turn the wine into colored water. The wine must be filtered at once to remove the flecks of bloom and then treated with one crushed Campden tablet per gallon of wine. The saved wine will have suffered some loss of alcohol and may need to be fortified with added alcohol (brandy works well) or consumed quickly. If caused by the mycoderma bacteria, treat the same as for a yeast infection. The Campden will probably check it, but the taste may have been ruined. Taste the wine and then decide if you want to keep it. Bacterial infections usually spoil the wine permanently, but early treatment may save it.
Prevent the introduction of spoilage yeasts and mycoderma the same way you prevent the introduction of vinegar yeasts -- by introducing early an aseptic level of sulfites.
Flowers of wine are, of course, expected when using flor sherry yeast. In such a circumstance, there is no way to know if the flowers are from the flor sherry yeast or a harmful infection. Pre-treating the must with Campden, however, should eliminate a harmful infection."
Here's the link if you want to see what else he has to say about winemaking:
also just be yeast.
If it IS mold, I've also heard that you MIGHT
be able to simply rack from under it & sulfite it in the fresh carbouy, but I've never done that, so I don't know for certain how well, or even if it would work.
Not sure about the sour taste, is it just tart, or is it more like a sour ale? It's possible that sour taste could just be young cider.
Hope the info helps. Good luck.