Polyp extension means the coral is trying to capture food such as marine snow that consists of bacteria, phytoplanktons or zooplanktons.
Polyp extension should not be view as indication of coral health. Actually, this means the coral is not getting the food it is needed at night and breaks away from its behavioral pattern seeking additional food sources in day time.
This could also be view as a way of corals competiting for survival in a captive system especially among the fragments of propogated corals. They need additional food source to secure itself in order to compete. HTH!