You can glue it, however, they have a slimy coating and they usually detach from it..The best way to propagate ricordeas is to place a rock next to them, and they spread, or, cut the cap off, devide it into 4 pieces and place them in a little tray full off crushed coral... you wil have to put them in a low low flow area...after a few weeks, they will have attached themselves to the crushed coral and you can then glue them with super glue, to the rocks without fear that they are going to blow away...
NotMuffy, Super glue or cyanoacrylate is commonly used in
SW aquaria for attaching coral
frags.. it is not harmful to the environment at all...