From Microcosm Aquarium Explorer
These barnacles are specialized to live on scleractinian corals and hydrocorals. They attach to, and then bore into, the coral. They secrete a single shell around themselves that is often covered by subsequent coral growth, so all that is seen is a rounded or conical bump on the surface of the coral with a slit or oval opening at the top. A typical barnacle filtering fan extends from this opening to feed on plankton.
Other common name(s):
Maximum length: 3 cm (1 in)
Minimum aquarium size: 38 L (10 gal)
Water: Marine 24 °C (75 °F) - 28 °C (82 °F)
Difficult to provide with enough food. Can be fed enriched Artemia or some other moderately large high-energy foods. Phytoplankton are too small.
They don’t really do harm to the coral they're attached to, although their activities may slightly weaken the coral’s skeleton and/or they may intercept food before it gets to the coral.
When the barnacle dies, the coral often grows over its shell and all evidence that a barnacle was once there disappears, except for a rather odd lump on the coral’s surface.