If you don't have a dremel you can use a sharp chisel, just place it between polyps on the mat and give a good hit with the hammer, also never frag anything in your tank and like mentioned above use gloves . Zoa's are somewhat tolerant of being out of water so I place the rock in question on a towel and do the deed quickly. I've also used a metal dish filled with tank water fir smaller pieces, be sure to shake off any debris and don't add the eater back to your tank as they are going to go into defense mode and slime
I only do this for zoa's or paly's and only for ones that are to big to cut. As mentioned above with zoa's toucan use a eazorblade to cut the mat and snip smaller pieces