That's an interesting one too. Hrm.
My favorite hypothesis (accumulated from various postings on Reef Central) is that it's a bacterial residue/side-effect of the vitamins present in most salt mixes. As they break down (1-4 days?) that's enough for bacteria to grow if the salt is left mixing "more than long enough". By the same token, that's one reason it's not recommended to keep your saltwater premixed for long periods - it becomes vitamin deficient. (Which is not to say it can't still be dosed with vitamins and used, but you've kinda wasted money at that point.)
Anyone have a microscope to examine some of the brown film close up? Should be pretty obvious under magnification whether it's mostly animal or mostly mineral.
-Matt
