The original clump is not going to grab on and pull itself towards the rock/driftwood/piece of whatever, but if there are strands in contact with it that aren't being moved around at all, any new growth off of those particular bits will cling to it. Then you can trim off the old clump. A little goes a long way if you can tie it down, though, because whatever is free floating above the piece and not actually touching it will be sending its new growth uselessly out into the water, whereas if all of it was touching, then all of the new growth would cling.