Hi, and Thanks for the question, which is a good one as quite a few members of the mussidae start out as attached (to a substrate) juveniles and then break off to lead a life of wandering the reef, Which Cynarina lacrymalis does. Sadly though, this can cause problems for both the Cynarina and other corals that can not get out of its way and can lead to stinging wars.
I have always found it best to take such free roaming species, with the exception of larger "plate" corals (which I won't keep) and glue them right back to where they came from in your tank. This can be done with super glue gel. It does not matter what brand. Simply bring the coral near the waters surface, turn it upside down and bring just the base of its skeleton out of the water. Then put a big blob of super glue gel onto that base and put it on a rock where you want it to stay. You will have to twist and/or rub the base back and forth a bit untill you see and feel the glue starting to stick to the rock, then just very gently push the coral down slightly (very gently!) and hold it in place for about 30 seconds and it should stay there. But from then on, be carefull because if you bump into the coral with your hand in the tank, it can break loose again.
Normaly I would not recommend these types of corals as they do tend to run into trouble when in an aquarium setting.
Chuck