I keep pushing my elegance further and further into the rocks and off the sand as it grows and utterly obliterates anything around it. I've had this thing since it was the size of a tennis ball........fully open now, with the pumps off, it's pushing football size! The tenticles do brush against the rocks, but it's a gentle flow and I have found it doesn't mind. At all.
I read in the wild elegance can be found in all sorts of environments from mud to rock. As long as the flow is gentle and can't tear or damage the polyp, I don't see any reason you can't put it in the rocks. You'll know pretty quickly if it's unhappy where it is.