toadstools, like any other leather, will fight you tooth and nail if you try to glue the leather itself.
best thing to do is either loosely yet snugly rubberband it to a rock, or take a plastic toothpick, lay it over the base of the coral, squishing the coral to the rock, and either rubber band the toothpick or glue the tooth pick to the rock.
leathers tend to slime up nicely if you mess with them, preventing glue from working well.