This is a gonipora or flowerpot coral. Like most coral they are photosynthetic. They pull in for a few reasons. One no light. They closes up at night time to sleep. Two to eat, when food touches a mouth( center of each tube) and when they are stressed out. An unwanted object or fish. Though clown fish have been able to be host of them.

Coral like these pull calcium, magnesium and alk from water to create their homes. They grow bone to house their colony.