What's that milky colored slime on the surface of my fish tank's water??!?

This morning I found a thin, milky-colored slime that covered the surface of the water in my 10 gal. tank. When I touched it with the fish net, it gathered up into a little slimeball. Gross! So then I scooped it with a cup and poured it down the drain. Anyone have an idea what it was? I clean the tank regularly and my fish are doing fine. If you know what it is, what's wrong with my tank?