If you just want to get the coral away from the Aptasia, you could pull the rock out and with a flat head screwdriver and a hammer, split the rock in half. Just try to make sure it won't fracture underneath the coral, do it as far enough away from the coral as you can.
On another note, if that's a Sun coral ( Tubastrea), it's the strangest one I've ever seen. No offense intended, but I think it was mis-labeled at the store. That looks like a deflated Carnation Tree coral. These are often called something else because they are considered to be an expert only coral, and most people won't buy them, if they know this. Most Carnations are nonphotosynthetic, if not all, and require feeding with foods designed for filter feeding inverts. Like live phytoplankton, and similar foods. They require low light and med-high flow.
Here's a pic of my Tubastrea, just for reference. Does yours look like this at night? Lights out is usually when they will open to feed, as they are NPS, as well. Although you can train them to feed on your schedule.
Sorry to be the bearer of bad news, but I don't believe that's a Sun coral.
Sun coral ( Tubastrea )