This is a product called "The Frag-Mag" by Aqua-Mags, LLC. (www.aqua-mags.com).
This is claimed to be a modular coral attachment system. I purchased them as part of a group by with my local reef club.
This is a magnet coral frag mount which you can move around as needed for example to help in adjusting lighting levels of new frags.
I don't think this is supposed to open up but this is the back side which goes on the outside of the aquarium. This would be the dry side of the magnet.
This is the inner part of the frag-mag where the coral fragment would be glued on. It is covered with some gray colored crush rock. I assume this is to provide a rough surface for gluing.