Most likely a protein/bacteria scum that accumulates from the heavy feeding these turtles enjoy. You can help keep this down by having the return flow from the filter keep the water aggitated. Once the surface stays likes this for a while, the filter can then process the stuff as it can get sucked into it. When the water surface stays calm the oily slick will return.
Also, a partial water change every few days (even if its clear) helps quite a bit.