370z Skid Skirts

[SvR370z]

My 370z is white and It went awhile without being washed due to the winter
and cold, wet temps we have.
I finally got up the guts to wash it saturday and for the most part she is as
good as new.
One area of concern is the side skirts. I had some kind of gunk on them.
It wasnt tar or paint and It would come off if you scrubed your arm off.
Does anyone have a product in mind that might help ease the removal of
gunk off of the sides?
I will clay bar the car and wax and such this spring but until then i want this stuff off as much as posssible.

Thanks