I used hairgrass in my 10gal with very low light, regular gravel substrate, no co2 and just dosed flourish weekly. DHG stayed green and alive but no runners and super slow growth. My tank where i use pressurized co2, high light, fluval stratum, florish and root tabs carpeted pretty quick. I saw runners in less than a week after planting. I leave my lights on for 12hrs a day
Yellowing doesnt always mean insufficient lighting. Could be lack of minerals/nutrients. I believe DHG needs lots of iron someone correct me if im wrong.
Try using crushed root tabs($5-$10), dosing liquid ferts($5-$10) and DIY co2($0-$5) are all cheap good options. You might not need them but theyre cheap and WILL help a ton IME with DHG.
For carpeting some tips would be to plant it small clumps and have them grow/fill in the spaces in between. Youtube and google search "dwarfhairgrass carpet" there should be lots of info on how to.