the best measure for you is to simply lift out that rock, and any other substrate it might be on as this tank matures, and rinse it off in a sink of saltwater poured over it use a light brush or even any triggered squirting device to dislodge. you can take myriad chemical and biological actions all shown to attack the colony
simple manual disallowance outside the tank is a powerful contender, and doesn't spread around bits as in tank work does, its true export.
your tanks substrate is larger than average and will take on a detrital loading 10x faster than an average reef, factor this, right now too new wont hurt.
in 6 mos? simple solution. or maybe a year, it ranges, but that's specifically an accumulative sand bed design and it will need to be forcefully vacuumed like a freshwater tank to avoid detrital loading, this is not like a typical reef where people do a hands off sandbed and the detritus will collect on top for siphoning, yours will go into the bottom of the tank. siphon it
occasionally, take everything out of the reef into holding buckets and take the whole sandbed apart and blast rinse it, put it all back, it isn't going to cycle anything. reacclimate the fish. do this as often as it gets cruddy, depending on fish waste incursion, feed, aging detritus etc.
always externally remove your invaders as a first reaction, we make giant threads on how that prevents lost tanks. giant ones. Its not that the chem or biological cheats are wrong, better, worse, its that among options forced reef control is cheap and the only true long term solution.
every mode other than this is a bandaid, since any invasion issues you have will be detritus driven/