If it is on a rock it doesn't have to work as hard to create a skeleton because it can use the rock but if it is on the sand it has to create the skeleton all on its own, so growth will be slower if it is placed on the sand. Be sure to keep any corals you like away from it because they have a tendency to kill any coral next to it!
^^^^ +1 on word of EXTREME caution in placing near other corals...they are silent killers by casting a web like structure over the coral which pretty much kills anything this 'net' touches. I have a couple echinatas in my tank and have had a couple casulties.
These like medium to low light, Sand bed preferable , a rock will help growth. These also like their space, its not adviseable to keep these close to other corals as they can kill the corals around them.