Weird Mystery Snail stuff

So I have 2 Mystery Snails who I thought were both girls living in separate tanks. I put them in a bigger tank together and they immediately started mating which lasted for a few days:shock: then the boy hitched a free ride from her for a while:roll:

I have one of the AquaTop that were on clearance from TruAqua. Great tank but one problem. It has an open area on the hood where you can do a HOB filter. I've been thinking of covering that up with some plexiglas and even had the piece cut to attach up there. But....Mrs. Snail crawled out last night and jumped. She was out of the tank for probably 16 hours or so. I picked her up, cleaned off the dust bunnies and pet bird feathers and put her back in. And closed up the hole.

She survived and about midday today I saw her laying eggs - but on the substrate. At first a small light colored eggcase came out then she exuded a bunch of black stuff. FYI the tank bottom is black sand. So why is she laying eggs at the bottom? Stressed out by the time on vacay from the tank? I doubt the eggs would survive in the water so I moved them to a plastic float. And what is the black stuff? More eggs mixed with the substrate?

Thanks,
Sally