If any of these are happening check your levels (alk, ca, mg, salinity, ph, temp) If any are out of range, bring them back into the appropriate values.
For stn or rtn infections. If possible cut off branches about 1/2" above the dying tissue line. Remove the rest of the dying colony from the tank. Take the frags you are able to make and give them an iodine dip (lugols) for about 10 mins. Use a container with tank water and put the frags in. Add enough iodine till the water color is light tan. Put your frags in and let them sit for 10 mins. Take the frags out wash them in clean saltwater and superglue them on a frag plug. Put them back into the tank with lower light and strong water flow. Give them about a week to see if they recover.
Coral bleaching is usually caused by either excessive light or temperature swings with the coral losing its zooxanthallae. Check your light schedule make sure it is working properly (ie lights arent running 24/7) and check your temperature range. Move the effected coral down away from any strong light sources. Give it about 2x less light then what it is currently receiving if the coral is under high light. If the issue is tank temperature bring the temp to around 77 degs. Reduce the lighting schedule to 6 hours a day for 2-3 weeks if you suspect lighting is the issue and watch how the coral responds. If it is going to recover you will notice dark spots in the coral tissue as the zooxanthallae try to recover in the coral tissue.