Two-part epoxy can be a great tool to secure corals to rocks or even rocks to each other, but the material can become very brittle and break apart when in use in the aquarium. Additionally, when mixing the two gel-like substances, a noticeable powder gets released into the aquarium, though this is usually harmless. To avoid having to use epoxy or at least the powder-like substance from uncured epoxy, pre-build all of the rock structures before placing them in the tank, or simply use other alternatives like acrylic or nylon rods. Super glue is also a decent alternative, or simply try wedging the coral frag into a crevice and monitor it long term until it attaches to the rock structure. Some aquarium keepers have even used the same material dentists use to make molds of people’s teeth. The possibilities are really endless, but whatever you use, make sure it is reef safe.