Any way to clean this stuff? Looks like a dark brown stain from the carpet backing. Not sure but in good shape otherwise.

Actually its this stuff.
http://www.lonseal.com/show_product/LONWAVE%20MIRAGE

Had black Pirelli floor in my Sonic and used to spray down with foaming tire cleaner/shine from the likes of AmorAll. Worked pretty good but also makes it pretty slick. Be careful.

i use either simple green or 303 as well as a steamer, it blows the dirt right out of the cracks.

Bleach, scrubbin bubbles, de greasers, etc doesnt touch wutever is staining this floor. its not a dirt or mold.

Cleaned ours with shower cleaner scrub brush,and wiped with dry rag,,,I think your right about the carpet backing causing the discolor

I have a couple small brown stains on mine. Nothing will remove them.

When you have your carpet made, you need to use vinyl as a backing. This will prevent the staining. Also, I noticed a lot of Florida boats flooring yellows prematurely due to the sun.

Yes, talked to Lonseal today. That is what they said, no rubber backing, vinyl only. Nothing will remove it. Have to replace. Looks like where the sun hit it, under seats are much better than the open areas.