What is the discoloration on my WRR's white plastic?

My 2015 WRR s showing some dark discoloration on the lower front part of the right side white plastic, just behind the frame. See pic. I'm the second owner, and have only had it a few months, but problem seems to be getting worse. Doesn't seem to come off with casual washing. Any ideas?

It's from the rider's boot rubbing it and grinding dirt into it.
Happens to practically any motorcycle, especially with white plastic.

Have you tried cleaning it a paper towel and contact/brake cleaner?

Of course! I should have thought of that (can you tell I'm new to riding dirt?). It's a little strange, then, that I only see this on the right side of the bike. Maybe the plastics are positioned differently on the left side? Maybe somehow I'm squeezing harder with my right knee? I'll take a look.

Thanks again.

You need the Magic Eraser.....

https://www.amazon.com/dp/B00BR1FSU8/ref=sspa_dk_detail_0?psc=1&pd_rd_i=B00BR1FSU8&pd_rd_w=nCcAh&pf_rd_p=80559f3c-f83b-49c1-8a72-40f936e9df7a&pd_rd_wg=NYd0s&pf_rd_r=AM6Z953NBRA01ZK0NT1Z&pd_rd_r=6a5ae4dd-5b9f-11e9-b52b-7f206e8c570c

Mine gets the same stain in the same spot. WD40 and some light scrubbing wit a paper towel cleans it right off for me.