What Caryl said x2.
Point a spray bar from the filter at the surface so it ripples and breaks up the oil/scum. Unfortunately this is a pretty standard byproduct of (esp.) planted tanks. It looks yuck from above but really doesn't do much. I'd make sure to suck away as much of it as possible on water change day and then the surface agitation looks after the rest.
Another approach, assuming you do have a planted tank, is to increase the fertilisers/co2 to combat the algae/scum by promoting plant growth. Since properly dosing my tanks with ferts and co2 I don't have algae and I don't get the scum. But it is a bit more work. Info on this approach is covered here - http://www.fnzas.org.nz/fishroom/viewtopic.php?f=2&t=50222