As Gus asked, what does your dipstick look like? If the oil is normal in appearance, you don't have much to worry about at this time. If the dipstick comes up like a light coffee color, then you really have water in the oil. Some water vapor in the crankcase is normal if the engine is not getting to full operating temperature. This water vapor will result in that small amount of slime at the cap.
As Joe says, you could have some seepage at the oil cooler. If you actually see coolant running down the block from the head gaskets, it is time for some new ones.