Could it be Filograna implexa Berkeley 1828, a polychaete flat worm from the Serpulidae family ? Or Seriatopora hystrix Dana, 1846, a stony corals (Scleractinia), or Acanthodesia savartii (Audouin, 1826), a bryozoan...
Diving, Fim du Mundo, Nacala, Mozambique, 2006