They can stay in the sand attached to something, or put it's foot in a hole or crevice in the rock.
I just learned a trick from
@Ron Reefman. Put them in a cap of some sort, bottle cap or PVC end cap, They will attach and you can bury this under the sand anywhere you want. Unless the condition is really bad there, they usually stay put. If you want them on a rock, choose rock with plenty of holes in it for their foot.
I got these yesterday. Did not get home until 11 PM. It was midnight before I got them into the tank. They were attached to the PVC caps a few minutes as I put them out of the back and put them in a bucket.It took me 5 mins to determine that salinity and temperature were the same. I just put them directly into the tank from the bucket without dripping.
I used 1 inch and 3/4 inch slip on PVC end caps. I used the 1 inch for the two that has PVC showing and 3/4 for the one that covers it. These are small but very colorful RFA under blueish light. Too bad I use white light in my tank. They are still beautiful.
This AM. barely 10 hrs in the tank, about 20 hrs from removal from their tanks at LFS (in Houston)