Attaching large coral colonies?

[Albertan22]

Thanks for the suggestions. I'm not sure what's with the rock. It's only the two pieces, the rest of the rockworks seems fine. The rock started out fairly flat and only maybe 2" thick. I know for a fact that the mantis that haunts my tank created a bunch of tunnels in there as well which probably didn't help matters. The rock is very porous as well compared to the others.