You can super glue it to the rock, I've super glued it in the past to driftwood with no issues. You can also use a luffa by taking the java moss and spreading it around the rock then wrap it with a luffa and tie the luffa down on the bottom of the rock that way once it grows in you won't see the luffa wrapping.
Fishing line is good if you are worried that it won't attach itself also, that way it won't disintegrate like cotton thread.
If you are using it as a foreground you can also use crafting mesh and sandwich it between two pieces then weigh it down on top of the gravel.