It looks like sponge in the picture. Did it show up fairly quickly or has this been a slow progression?
There is a fungus that attacks zoas The fungus resembles sponge in appearance. Can you remove the rock for inspection? If fungal you can use Lugols Iodine, hydrogen peroxide, or Seachem Reef Dip.
If sponge, you will need to move the zoas or the sponge. For fragging zoas I like to use wax carving tools (can get on Amazon for around $12 for a full set). The spatulas are perfect for gently get under the zoa and removing it.