they generally only move when they're looking for that right combination of light & water movement. If you have a spot where you would like it to be, move it there and play with your power head a little - get some decent flow but not too much (this is assuming that your lighting is strong enough).
Oh yeah, when they look for a place to stay, they will generally move to an area where 2 rock come together to squeeze their foot in there - they always try to keep it hidden