I would never put a clam on the rock work unless its already attached to a rock rubble.Once they attach themselves its nearly impossible to remove them.
Sorry,never heard of croceas prefer the rock than sand.In a matter of fact,I never heard it either way.You can have it on the sand or rocks depending on the lights you have.I prefer the sand,it just looks more natural to me.Underwater,you have the lights so the sand or rocks will work.Start low and work your way up.