It's not uncommon to find this sort of film growing in barrels if you leave them too long between topping and a large headspace forms - and your wine is also relatively high pH, which also can promote unwanted growths.
The best way to get rid of it is reduce the airspace and maintain SO2 levels. When I'm topping a barrel that I know has film, I always 'overtop', filling the barrel through a wand that's submerged under the surface and adding wine until it flows out of the top. No airspace = less chance of spoilage microorganisms. However, if this is all your wine in this container, topping might not be an option unless you use, eg a commercial wine of the same varietal to fill the headspace.
SO2 is also your friend. If you have things growing in there, I would bet most of your SO2 has been consumed so it's good that you added more. (How much, 50ppm?) And hopefully that tartaric add will bring down the pH and discourage further growth. Did you test the pH after addition of the tartaric?
Finally, are you saying that you sprayed the top of your wine with StarSan? Not sure about that, I've only used it for sanitizing empty vessels and hoses. I would spray with, eg 6% w/v KMBS instead.
