You are going to get algae, all new tanks do. Wait until your cycle is completed before adding snails/fish/anything else to the tank. Your levels shouldbe close to the following:
Ph 7.8-8.3 (higher end there is better)
Alk 8-11dKH
Calcium 400-450ppm
Nitrite 0ppm
Nitrate 0ppm
Ammonia 0ppm
Phosphate 0ppm
And theres magnesium and strontium which I don't test for and subsequently do not supplement into my tank. do not add chemiclas that you cannot test for.
But you haveing a newer tank, don't expect your levels to be this optimal. That takes a little time and effort. As long as your Nitrite and Ammonia are 0, that's good. Nitrates should be as low as you can keep em, anything over 20ppm and I would worry a bit about your system and maintance. Phosphates should also be kept as low as possibly. They will fuel algae growth and since you'll have some algae from being a new tank, this can make it alot worse.
As for the snail, if one dies and you don't see it, than you won't know.

Just remove the dead ones that you find, otherwise I wouldn't worry too much about em. Alot of snails have short life spans anyways and you'll be restocking some every 6 months to 1 year anyways. More so if you have hermits in the tank, little punks always killing my snails!