These corals completely lack the symbiotic zooxanthellae required for photosynthesis so they do not need any light whatsoever. In fact, they might not even want any light at all. I've had a dendro for nearly a year now and it sits completely under an overhang, no light reaches him at all. he is quite happy and open all the time, night and day. If I recall correctly it did take him a few weeks to get used to the tank though. He was closed for a good 2 weeks I believe. Give him some time maybe and try moving him into a lower lit area. Also, even though his polyps are closed up he can still feed. You have to spot feed them at least two or three times a week. Get a turkey baster in there or tongs and place some food into/near his mouth. He can still eat it with his tentacles retracted.
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