"Oily" substance on top of water?

I only run my filter for a few hrs every day because it makes my frog go mad.. Swimming around constantly, can't relax, etc. It is the only inhabitant in the tank.. no water breathers. The only reason I do turn it on is because there's always this oily substance on the top.. I do water changes 1-2 times weekly. I use water conditioner. My frog doesn't seem too affected by it, I was just kind of wondering what it is?