Looks like the grease from heavy machinery. Guy who plows my driveway left a nice patch of that a few years ago. Certainly not done on purpose. A good reason not to be skinning resorts in the off season--even if there isn't machinery operating there is plenty of dirty snow. Good thing it's on your skis and not your skins.
Mineral spirits are as good as anything for getting off grease after you've scraped off the bulk of it.
Why not us Toko Wax Remover? 4 times the cost.