I can't answer for certain since I can't test and sample it, but I can tell you of my own experiences with mead going off:
I've had mead go off that looked fine, smelled only a little bit odd, and was so awful tasting that a couple of drops on the tip of the tongue triggered an involuntary gag reflex.
My girlfriend once had mead that went off and there were little black patches that formed on the surface with long thin tendrils drifting below. Probably some kind of interesting fungus, but we still dumped it (and no, even though it didn't smell off, we still did not taste it).
I've seen films form on the surface, usually earlier in the fermentation stage, and in my experiences they usually fade (maybe the stuff they're made of settles to the bottom over time). Mind you, I don't rack to secondary (I usually just bottle after a few weeks in the primary), so I've never had conditions quite like yours. I'm really not sure what caused this film so late in the process.
If it tastes and smells OK, it's probably safe.