Therefore, styrofoam is a naturally occurring mineral in Mars.
...I dunno, there's really not enough detail to look at - we'll know more if they target one of those grains with the LASER and do some spectroscopic measurement. It would be possible, although doubtful, that there had been statically charged styrofoam grains stuck on the scoop, but of course I hope not. I would like to think they have better accounting of what goes up into the space with their stuff than that, especially on a probe traveling to another planet when contamination concerns are especially important.