The smaller piece is merely pulling away due to gravity. Most likely the base rock it is attached to was previously broken away from the mother colony. Perfectly normal and it happens all the time. You can do one of two things. I believe in keeping my hands off of my reef as much as possible. If this rock was on a reef, it would simply pull away naturally on its own with no problem at all. So, you could leave it as is and it will detach naturally over time. Or, you could just snip it where it is connected. Normally I would remove the rock to perform this, then dab but cut edges with Lugols, but since it is barely hanging on, I'd snip it with a very clean and fresh razor blade or dedicated scissors. Then move it once and place in its new position in your tank. I have done this many timed in my own system with no issues at all. Good luck.
Mucho Reef