Ricordia is very good at repositioning itself if it has a chance. It would be difficult if was buried to do so but I have had ricordia flip around with ease so position the rock to benefit the clam. Even if you prop the rock up just barely off the substrate, the ricordia will eventually right itself. I have done this quite a few times to get a rock positioned just right with the ricordia on it. I also put some flatter pieces next to the rock, just in case the ricordia want to move off the original rock. I realize you don't want to risk flipping the rock so make sure the original rock is stable. Hope this helps and hey congrats on the baby...clam that is!
Sherm
Sherm