That's urine, and the white substnace is excess calcium deposits left after the urine dries. That's normal for rabbits, unless the calcium deposits are thickened and creamy, gritty, or gel like, then that indicates a problem called bladder sludge, and means changes need to be made.
It actually is usually strongly recommended to get non breeding female rabbits spayed, due to it helping eliminate hormonal issues, as well as due to the increased risk of uterine cancer for female rabbits. If you have a male, then obviously it would be to prevent pregnancy as well. If your vet is telling you spaying pet female rabbits isn't necessary, I would suspect this vet isn't particularly knowledgeable about rabbits. Spaying female pet rabbits is almost always recommended by knowledgeable rabbit vets.
https://wabbitwiki.com/wiki/Spaying_and_neutering_rabbits
I would suggest consulting with a more experienced rabbit vet, to get your rabbits properly sexed, and to discuss spaying/neutering. They are at the age for those hormones to start causing hormonal behavioral issues and hormonal marking. Getting them both fixed will usually help improve all of these hormonal behaviors like urine marking and excess humping. Plus if you want to keep them bonded together, getting them both fixed seems necessary at this point.
https://rabbitwelfare.co.uk/rabbit-...abbit-friendly-vets/rabbit-friendly-vet-list/
If you haven't separated them into different enclosures and plan on keeping them together until you talk to a knowledgeable rabbit vet, just be aware that there is a risk of aggression and fighting, particularly if you do have a male and female, and your female is now pregnant. So please keep a close eye out for signs of aggression escalating, and separate immediately before things escalate. Though my suggestion would be to separate now, keep in enclosures next to each other so they can still see one another, then get them both fixed before attempting to rebond.
