I will paste the reply I was given from an expert in care of lobo. With high experience with wild caught / imported specimens
Hello,
Based upon the photos that you sent, the Symphyllia is indeed bleaching, The first thing that you want to do is inspect the edges of the coral & the bottom of the skeleton to make sure there are no razor clams or pests like worms, vermited snails or anything else that could possibly bother it. You can also dip the coral to see if anything comes off, preferably in iodine or Revive so that the coral does not stress out more. After dipping the coral, place it in the darkest part of your tank for a couple of weeks or until the color starts to come back & don’t forget to target feed it so that it recovers faster.
When a coral is bleached, even medium lighting can cause it to stress out and continue to bleach. In a couple of weeks, if you see the color return, you can slowly introduce it back to the light.
Lobophyllia & Symphyllia will typically have Razor Clams inside of the skeleton. These clams will die off & when this happens, the gases from the rotting clam escape through the flesh of the coral, causing it to bleach. I also notice that you have your Symphyllia in the sand bed. Sometimes, fish or a pump can stir some of the sand onto the corals & then the sand can also cause the coral to bleach if not removed in time. This is because the coral will ingest the sand, thinking that it is food. You can buy a turkey baster and blow the sand off the coral to keep it clean if that’s the case. Another possibility would be that it got stung by another coral with long sweepers that is nearby.
Medium lighting and medium water flow are ideal for Lobophyllia & Symphyllia.