Here’s a close up picture of an Acropora coral. A few of the polyps can be seen coming out of the corallites during the day. At nighttime this coral, like most, will further extend its polyps to try and capture free floating food. If there are pests like red bugs or AEFW present on the coral’s tissues, it may keep its polyps retracted both day and night. This also holds true if coral nipping fish are present in the aquarium.