Best and safest bet is to soak overnight in distilled vinegar 1:1 ratio with water. Toss a powerhead in there. Next day remove it and scrub the rock with a brush to remove the dead bits. Get a big pot and boil the rock outside. Palytoxins are no joke, never ever worth the risk of you, your family, or your pets.
After boiling let them dry up in the sun for a few days. Then you can cycle them.