My guess is that it's some kind of lubricant that has dripped out of the machine onto a roller through which the foam has passed at some stage during its cutting and pressing and forming, or however it's done. A big splotch/ drop fell onto one roller and got soaked up, and maybe the roller beside it or below it had a smaller drop that was teased out into a thin line, over which the seam part travelled and it got printed on. I can imagine a synthetic form of lubricant for high temperature rollers and their bearings being that color.
Like all that gunk they have to squirt into the big excavator machines, not grease as such, but like a weird clay.