Mav:
I found out about a month ago that by lighting the water, you slightly charge it. The charge attracts gunk and dust, which is what you are seeing. There isn't really a way to completely prevent it, but you take steps to minimize it once it has occured. Mollies feeding off the surface, paper towels, surface skimmers, etc. are all options, but the fact is that white gunk is endemic to the type of systems we are running.
Now, having said that, I used to rely totally on trickle filters to combat white gunk. Since I have had to cease with the skimmers due to invertebrate deaths (lil' punk amanos would climb over the overflow, get into the wet/dry and die without food), I have noticed an increase in white gunk. I do have some surface motion which tends to minimize the gunk, but not enough to destroy or remove it.