That's probably a protein film; they're pretty common in tanks with little surface agitation. As far as I know they're harmless to turtles.

Saltwater aquarium enthusiasts use a device called a "protein skimmer" to deal with this problem, but I don't think they work in freshwater systems. An air bubbler, waterfall, or water spray can help, anything that disrupts the surface. You can also just use a fine mesh net to scoop up the gunk periodically and dispose of it.

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