Stony corals are anthozoans, with polyps bearing tentacles in multiples of six and no medusa stage. Millepora is a hydrozoan. It secretes a calcium carbonate skeleton, hosts symbiotic algae and contributes to reef structure, which is why it is grouped with corals in ordinary speech, but its anatomy and life cycle are those of a hydroid.
The surface is pierced by two sizes of pore. Larger gastropores hold feeding polyps; smaller dactylopores hold defensive polyps whose fine hair-like extensions carry potent stinging cells. Those threads are what a swimmer brushes, and the pain is immediate rather than delayed, often with a raised welt lasting days.
Millepora also releases short-lived medusae from chambers in the skeleton, a stage stony corals do not have. Identification in the water is genuinely difficult, because the colony often overgrows sea fans and dead coral, taking their outline. The smooth mustard-yellow surface with white branch tips is the practical field cue, and the safest habit on a reef is to touch nothing.