Dwarf hairgrass is a great foreground plant for those looking for a taller cover than traditional carpet plants. It spreads readily via runners when kept under good light with proper nutrients and can become quite invasive unless tended. Due to its fine leaf structure it does have a tendency to attract algae, so water conditions need to be kept in line. Trimming the shoots will promote faster lateral growth via runners.
I thought to develop this as a carpet early in the tank's life. I recommend making sure water conditions are great first as I've seen diatom, hair, and staghorn algae attack and all are annoying to remove from the hairgrass.
I think getting a successful and quickly grown carpet with this plant involves high nutrient substrate. I had this in a high light pressurized co2 tank with just eco complete and seachem root tabs and it has hardly spread after months. I put some in a newly set up tank with diy co2, medium light, ecocomplete and MGO and it started spreading within a week.