You don't have fish and you don't add fertilizers but your nitrates are 20? Are they coming from your tap water?
You should run your lights only 8 hours, and add a comprehensive fertilizer. Seachem flourish in combination with API leaf zone gives all the macro and micros you'll need.
With low light, you really don't need CO2, but it will help plant growth and preventing algae.
I hate to be the bearer of bad news, but that hairgrass is a very high light, high maintenance plant that you probably won't have much luck with getting to carpet :/
With your light, try plants like anubias and crypts in addition to your java ferns.
The white growth looks like a mold or fungus to me, but when all the other things are balanced in your tank it should go away on it's own.