Oil Slick on Beer

The only two batches of beer I've made so far have this oil-slick like appearance on top of them in the fermenter. It's not over the whole surface, but broken up. The first batch turned out fine taste-wise, and this second batch is still in the primary but I can see it on the surface again.

I'm brewing with malt extract and steeped grains. For the last batch I used Safale 06 (I think, it's the wheat one) and for this one Wyeast American Wheat.

Can anyone tell me what it is?

Sometimes it's hop oils, could be your water too. I know I'd never brew with my well water. Yuk.