Dont worry about the dark bits on the java fern, new plantlets will usually grow from them, that's what the bits growing out them are.
Snails pick out as many as you can, they will decimate your plants and become a problem. I know where you got your plants its one of the worse places in the area for buying from, their fish and plants are seriously crap and they push chemicals at every chance they get. They are in it simply for the cash not the fish welfare and the advice they give is even worse at times. I got dirty looks for telling a woman that despite them saying a 21litre tank was ideal for 3 goldfish it was totally wrong and her fish would be dead in not time. They told another that an oscar was fine in with neons etc. There is one place worse which I will tell you about when I am down next. I will come down through the week and have a look and bring the test kit down and check your stats again as you were not too far off when I tested it last week.
Don't add the algae treatment especially while the tank is cycling. As for the sand just give it a slight stir, run your fingers through it. When it is finished cycling you will have to do a large water change which will get rid of most of the white bits but without seeing it, its hard to say what they are but I suspect its a bacterial bloom which is really common in a new set up and isn't anything too worrying. You have nitrites in the tank just now which add to the problem with algae, once that part of the cycle is complete it will slow the algae issues.
Stop worrying its not going wrong lol.
