There are several brands of coral / rock work epoxy on the market and now there is another option called CoralCrete from Seachem.

Seachem CoralCrete™ is two-part epoxy putty that can be used to secure coral to rock work or fuse rocks together for a more stable aquascape structure. Like most epoxies, CoralCrete has a working time of 5 minutes before it begins to harden to the point where it is not usable. CoralCrete™ is ideal for working with larger colonies of branching coral, many large polyp corals or adhering plating corals (like Montipora caps) to rock work. It is safe for aquarium inhabitants when used as directed. MORE