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.