Head gasket?

I was checking my antifreeze today and noticed a small amount of a tanish kinda oily substance at the filler neck. It was not much, the antifreeze is full, and perfectly green. I swiped it off the sides of the neck with my finger, it didn't smell oily, but a little slick when I rubbed it between my fingers. No water in my oil whatsoever. 1993 Olds Cutlass Ciera 3.3L.