Have you always used wood chips as a substrate? I've never heard of using that. ^^;

I'd stick him in a quarantine tank with paper towels and a water dish and see what happens. I can def see the film and it just does not look healthy. I dont have cubans but we had one at work for almost a month and that just dosn't look healthy. Id probably give a call to a herp vet if you have one near you or maybe the local zoo might have a frog expert.