It is better to use just the newspaper with black and white print. Was once at a gardening seminar and one of the presenters made a blanket statement about using any newspaper for mulch. I asked if it was ok now to use the color pages. Both presenters got reticent quickly. Some metals are used in those hues. We don't need that quantity of those metals either in our gardens, salad bowls or fishtanks. ;)

I've read elsewhere that it can be a community of algae, bacteria and other organics. It may be a hint that organic goodies are accumulating in the water.

In Bob Steinback's 4,000-gallon pool in the floor of the biology wing of Elgin Community College (IL) there is an observable film. In that case the film is fueled mostly by skin cells shed by passing students and faculty. Someone here started teasing about Soylent Green and Miskairal almost skipped lunch. (I think her initials were Angelhologram.)

Black mollies and even guppies, if they are hungry enough, may eat that white stuff. It may form if too much rich (frozen and live food) is used. While iron is needed for plants, too much of it, according to aquatic gardeners, also may trigger that white film. Darn, we might want to do another partial water change. I'd skim at the top. Somewhere there is an old wide, straight edged filter box. That might suck it up pretty well.

Vigorous filtration will break it up. Of course it is just getting absorbed by the water again.