I think that's not it but its plausible.
http://www.thekrib.com/Plants/Algae/surface-scum.html

Surface scum is primarily composed of water-insoluble organic
compounds (proteins, fats, aromatics, etc) from fish food, and/or from
the decay of plant and animal matter in your aquarium. As the scum is
both water-insoluble and lighter than water, it floats. Secondarily,
other things can colonize the scum (presumably while feeding on it),
such as bacteria, fungi, and, of course, springtails.

It seems to be more than one source food included.

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