The algae is probably just a matter of too much light considering how few plants there are and how young they still are. They aren't using nutrients very quickly, so algae moves in. At least the snail is interested in it.
I love the water onion bulbs, but I have started buying the tube plant versions from PetSmart - they're all ready partially grown so they will take less time to establish. Bettas love them, and I've got one that's around 30" in a 5gal hex with a male halfmoon. It's long and sturdy enough that he can use the leaves as hammocks instead of the requisite plastic betta leaf, lol.
I say that it was weird mostly because, if you look at the one stem, it literally just cuts off. Like it was dipped in something <_< It was just an odd infestation; my snail's gonna get it taken care of pretty quickly, though, I think.
I've got one of the aponogenton bulbs that grew like a weed once it was established, but this is the first water onion I've gotten, so we'll see how it goes. It sprouted, and that's half the battle!
I've come to realize that Petsmart and Petco have vastly different things available--and I only have access to a Petco, plus my LFS, which deals mostly in the common anacharis, anubias, vals and java moss. You can get some neat stuff there sometimes, but I have a feeling that's mostly from people who donate things, not what they get in normally.
My betta liked this one plant I had that I actually let float because I didn't have anything to tie the rhizome to--a bolbitis heteroclita(?--I never got a 100% solid ID)--that I'm learning has trouble with immersion? >_> Bah.
Edit: Snail video
. It's a video of a snail.