If a coral is stressed/bleached, it will need to be fed actual meaty foods, as it cannot "feed" via it's symbiotic algae (Zooxanthellae). It will also need to be kept in a shaded/indirectly lit area while it recovers.
Trying feeding it small pieces of any raw seafood (scallops, clams, oysters, etc.) soaked in a good marine vitamin (Selcon, or Zoe are both good). The best time to feed an Open Brain is later in the day/evening. Just gently tuck the food into it's "folds".
You don't have any dwarf angels that are picking at the coral, do you?
Here's a good article to read:
>>The Tragedy of Artificially Colored Live Corals<<
-- yes, I realize the article is about dyed corals - but in the article, there is good information on how to bring these corals to recovery. Since dyed corals have lost their symbiotic algae, their care should be similar to what your coral requires.
Another good article: >>Acclimating Photosynthetic Reef Invertebrates to Captive Lighting<<
One more, specific to care of the Open Brain coral:
>>Open-Brain, Rose Coral, Family Trachyphylliidae<<