I agree that they wont attach that well to sand, so you don't have to worry too much about them migrating around that way. However, do you have anything like an urchin in your tank? I have two variagated "carrier" urchins in my tank and they have physically transported some ricordia mushrooms to other parts of my tank, which have started new colonies.
I snapped a photo the last time an urchin was moving one:
