I'm losing it from the comments.
Did someone just nut on the MC at the last panel!!
That's gotta be saliva from the ahegao faces, or maybe some ladies came so hard they started lactating and no nursery pads were present.
If we really gotta stretch it, then maybe it's vaginal fluids from women who were wearing shorts or skirts, but I'm having a time imagining the positions they would have to be in for that to happen to Furuya, considering he was running in a straight line, and has to make physical contact for his power to work.
And guys cumming through their pants... to leave Furuya messy like that? I'm really doubtful of that one.