The layer of scum makes me think your brew might have gone mouldy or have a bacterial infection. If you're lucky it could just be some floating detritus has gone mouldy and the stuff underneath is okay. Try removing the scum and tasting the wine - if there is any hope of flavour then your idea of racking and adding campden tablets (I'd use 2 per gallon, twice the normal) and see if this works.
If it's on the way to vinegar then help it along some more! The best wine vinegar I made was from a kit wine that tasted awful. Rather than throw it away I divided the demi-john in two and poured each half into a new container with an inch of vinegar on the bottom (to start it off) and plugged the top with cotton wool. The vinegar and air in the demijohn gave me a really fruity red wine vinegar, the best ever chip coater ever. I think basically the kit wine was under nutriented and needed aging, the air in the vinegar demijohn acting as premature aging and helped the flavour.
If there is no hope use the brew as a high nutrient addition to your compost heap.
