Those are urates, not "man juice" .... like Jas says, typically females will evert their scent glads and drag them around the tub trying to entice a newly introduced male ... sometimes as they do this, urine and urates are excreted and often, the female will roll her body or twist their tale in them (possibly to spread the pheromones that are mixed into the urine onto her back?) ... it's not a 100% for certain sign of a lock, but it's a good sign that your female is receptive and if the male is producing sperm and they were laying together when you found them a lock was likely.

One "trick" that I like to use when I see "signs" like that (urine smeared around, blood trails on the paper, etc) is to pop the male ... if they have been copulating, usually one pene will come out a little easier than the other and will be slightly darker and sometimes even more swollen (a sign of recent use). Often you'll see one pene with a sperm plug and the darker more swollen one without. For me, that's a definite confirmation of a lock.

Congrats Jake!

Hope this helps.

-adam