Those are not white walkers. They appear to be a petroglyph, a citrus cooler or most likely some sort of vampire morph (there's a lot).
Its a large type zoanthid so that means a few things 1) they will spread reasonably well, 2) they'll usually do fine in lower (or higher light) and 3) they'll benefit from broadcast feedings.
My recommendation would be to put it on your sand bed and keep it off your rock work. As it grows and matts on the sand bed its super easy to trim back and trade/sell/give away.