Veretillidae

Coconut Garden, Lembeh.
This is one of many sea pens found in the night dive all over the sandy floors. It is very difficult to find sp. for these. All I can say is sea pens are octocoral colonial cnidarians owing their name to their brush-like appearance.
From Wikipedia:
Unlike other octocorals, however, a sea pen's polyps are specialized to specific functions: a single polyp develops into a rigid, erect stalk (the rachis) and loses its tentacles, forming a bulbous "root" or peduncle at its base. The other polyps branch out from this central stalk, forming water intake structures (siphonozooids), feeding structures (autozooids) with nematocysts, and reproductive structures. The entire colony is fortified by calcium carbonate in the form of spicules and a central axial rod.