Ive got four in my nano which is open top and its never been a problem. I did find a few outside the tank when I first got them but popped them back in and they've not done it since.
The white egg thing can be a real pain. I seem to have 7 males to the mo which is very lucky. If necessary you could keep them in separate pots for a few days to see if any are females. They aren't that expensive.
Or just check the shop tank, if they've been in the shop tank for over a week and there are no eggs then its a safe bet that they are all males. Some suppliers only sell one sex of a fish to stop people breeding them themselves. It could be the same with nerite's not that they would be easy to breed with the larvae needing brackish water!
I couldn't be more pleased with mine, I'd have them in all my tanks if it wasn't for my loaches, keep both tanks, more or less spotless. Pretty little critters too.
I've seen mine eat the thin, almost slimy thread type algae, took them a few goes but they gobbled it up in the end!
Sam
Sam