Ahh I know what you're all on about now. I had some of this when I first set up to combat the early "algae growth" stage. I went away for a couple of days and it just took over!
HC - hemianthus callitricoides, a great little plant, bit like glosso but smaller leaves and more of a trailing stem that grows along the gravel. Would highly recommend it if you have enough lighting. Also needs plenty of NO3 I keep mine at 20ppm and that seems to get best results.