Looks like the base is dead. You can cut off that bottom part and re-glue it down. If your lighting and tank params are good it should be healthy and start growing for you.
Bone cutters, wire cutters, needle nose pliers, just to name a few... I don't believe it is diseased however I do know that they are intolerant to soft coral toxin, need good water paramos and sufficient light. Just a heads up.
It looks like the coral is fine but something happened to the base... just like others have said cut the base and reglue it giving it a chance to grow its base. it will do alot better this way. You could also use a tile saw if you have one.