Pmd on dry rock. I would setup a stock tank and get as much rock seeded with bacteria as possible. Bacteria grows expontentially so the quicker you start the better. I would consider getting a ball of chaeto from a fellow reefer if you are worried about the caulerpa taking off again. I believe mortar has to sit for awhile as it is pretty acidic at first. I want to say a couple months but that is from reading about making your own rocks way back when I started reefing so its probably not right.