It's dye transfer from your jeans. It's a pain in the ass to remove. I have used Leather masters products with good results. Not sure if Corey sells something that can help. Perhaps he can chime in.
I will say regularly using UIGP really helps reduce this issue.
We have some things to avoid this situation but not a lot designed specifically to remove it. On finished leather I have used eraser followed by immediate use of uwwp to remove any strong cleaners from the Eraser.