tatami help needed

I have been a bit busy the last little while I haven't really had time to do some proper cleaning in my apartment which has tatami floors. So now that I have time here I found some kind of weird discoloration on my tatami. It came from a extra tatami top that the tatami guys gave me to put my bed on. I clean off all the junk left behind from the mate that went under the bed but I can't get the discoloration/stain out.
Any advice out there on how to fix this ??

Vinegar work sometimes or try cleaning product called "Shin Tatamigae".