Id definitely weigh more on the alkaline or bleach way of stripping the organics off the "rock" and would probably then acid wash before cycling again.
I've got a rock with palythoa on it, it was my first ever coral and has been near indestructible across numerous tank moves and occasional disasters.......I'm just weary of it having the potential to do me harm and I try my best to just leave it in piece and fish out any that's trying to escape their rock.
As to the initial quandary of the disappearing Alk, I can only assume because it was a fresh piece of ceramic that it had some sort of impurity, maybe aluminium etc. that could have caused a reaction or even the fresh surface offered an ideal starter crystal for abiotic precipitation? Baring an erroneous test result?