You would think it’s some kind of wacky secret but it’s just glue.
Use a towel to dry a wet live rock rubble really well, add a dab of glue. Dry the underside of the macro really well, add a dot of glue, and rub it in a little with the nozzle. Press the two together, dip in sw. Bonded!
It does not seem to grow ANY kind of holdfast, but the glue holds for a while. It will probably come loose eventually, and it might just tear at the base when it gets too big and catches the current.
I highly recommend growing it glued, it has a really nice foliose growing pattern