Mold or not?

Good day! A couple of days ago I bought a book of the 39th year at auction. In general, the condition is good… https://pp.userapi.com/c845419/v845419777/1d072c/lN-ql6iN-Xc.jpg
but on the cover was something like black or dark gray powder. https://pp.userapi.com/c851320/v851320972/e0a79/9XGf2PcS94M.jpg
I did not immediately attach importance to this and wiped it with a napkin. On the cover are visible shiny traces of whether processing, or something spilled.
https://pp.userapi.com/c848732/v848732777/1617ba/l490vXxaqlE.jpg
There are also many small brown spots on the cover, but the pages are completely (or almost) clean.
https://pp.userapi.com/c846420/v846420814/1cdcde/bC2lhBoexDc.jpg
https://pp.userapi.com/c855724/v855724137/8387/nfRV5lw7kSA.jpg
For the sake of experiment, I wrapped the book in blank paper and observed whether new disputes appeared on it, whether contamination was taking place. On the one hand, I didn’t see anything terrible, but on the other, a strange dirty strip appeared on one of the sheets (and small, tiny spots on the other).
https://pp.userapi.com/c851036/v851036972/e0864/BmlovOHVPzo.jpg
And the strangest thing is that the strip is on the reverse side of the sheet. Not the one that fits on the cover. And on the same place from the adjacent side the sheet is clean. I have time until March 26th to return the book. And I do not know whether to try to clean myself, and how to understand at all whether it is mold or not. The cover of the book is soft, dressed in a magazine dust cover. I hope that the images on the links will work. I will be glad to any advice.