Hemianthus callitrichoides, HC, not an easy plant to keep down. If you have active bottom dwellers they'll knock them loose.
Some people separate the HC into individual strands and bury them into the substrate with a tweezer. The tedium with that method would drive me batty, I break the HC into tiny clumps and bury them, again with a tweezer. A fine grain substrate helps tremendously.
If you haven't added water to the tank yet there is a MUCH easier way to grow HC, though it does require patience. In an unfilled tank add the substrate and fill with water to just under the substrate line, you want your substrate soaked but that's it. Then break the HC clump into pieces and just toss onto the substrate, no need to bury. Cover most of your tank to retain moisture and add ferts when you mist the tank to re-humidify. In about a month you'll have a nicely rooted HC carpet and can then fill the tank.
