I bred bettas at one point, and in each juvenile container I had a huge film problem. It's a 'protein film' and aside from filtration, I don't know an easy solution for it. It could even be caused by the betta's protective mucus kicked into overdrive? It's hard to say!
If you have a filter and your tank is cycled, emptying it out frequently and washing everything will kill the beneficial bacteria though, and cause it to cycle again.