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If you feel your skimmer is working well (producing decent quantities of dark skimmate, appropriate for the biomass and feeding level) then I would think a good possibility is some sort of bloom or spawning event. Whether or not shrimp could cause this- not sure. I find that surface films are uncommon when sufficient skimming is taking place. Did you have any animals die off recently?
I'd recommend to manually skim the sump- and do a water change to hedge any potential problem.
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