Harmless sponge. If it does seem to be bothering the zoas you can take the rock out of the water and let it sit out of the water for a minute or two. The zoas will be fine but sponges die when exposed to the air. You could also get a toothbrush and scrape it off pretty easily.
I had this same type of sponge all over some scrambled eggs zoas and it looked like the sponge was killing them. Turns out there were 5 large spaghetti worms that were intertwined with the sponges and zoas causing the zoas to remain closed all the time.