When I get a free-floating ricordia, they tend to float around in the aquarium and end up somewhere they are unhappy. So I take a very small bowl (sort of like a condiment dish or salt cellar), add a couple teaspoons of the sand in it, and then put in some rock rubble on top. Then place the ricordia on it. You should place it somewhere in the tank where it gets some flow, but not a hurricane! I put mine in the mid-light level area. If the ricordia continues to float out, place a small netting top over the dish and secure it with a rubber band around the dish. (I use on old laundry net- the kind you put delicates in to wash.) Then give it a week or two, and the ricordia will pick a piece of rubble and attach. Then you can tale the ricordia out of the boawl and place it in your aquarium. I also feed the ricordia while its in the bowl just in case the netting keeps food from getting to it.
This method works for strays zoas too.