If you kill the sponge by drying it out, it will release its toxins and bacteria which will cause a white fungas and infect the mat of zoos on that rock, which could kill them and infect other zoos in your tank, as the bacteria spreads.
I would just take an exacto knife and scrape as much of the sponge off as you can, or peel your zoos off. You wont hurt the polyps! good luck