My Dendro opens and closes all day but if I neglect it then it will prefer to open only in the dark. Check soon after lights out and before lights on to see if you can catch it open and when you do find it open feed it small pieces of meaty food.
This coral requires lots of attention as it needs to be hand fed every 2-3 days. If your critters are stealing the food then you need to chase them away or use a strawberry basket type solution. When healthy my dendro envelops the food in about 30 seconds so there is not much problem of the critters stealing the food, I feed mine mysis and enriched frozen brine shrimp.
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