I'm out west and we get this here. Only in late spring, hasn't started too bad at my mountain yet but it's only a matter of time.
I'm not sure that it's actually "grease" but it's black sticky shit that adheres to your skis. You ever look at the snow piled up on the side of the road after it hasn't snowed in a week? Notice how its gray or even a nasty looking black? Well in the late spring you're just skiing on old snow pack and the snow gets dirty as fuck. The groomers kind of scrape some clean spots for you, but they cut back on grooming in the spring and at some point you are going to be skiing not on the narrow groomer strip, and then other people are dragging that black stuff onto the groomers, your skis end up getting these huge black tar looking spots all over the bases. I'm guessing it's just fine dirt and dust, tree pollen, lift grease etc anything that can be carried on the wind and it mixes with the oils and old wax already in the snow.