Yellow 'christmas-tree
      worm' on an orange coral.

A yellow Christmas tree worm

Yellow christmas
      worm on a white coral
Sea worm lives
on a white stony coral

( Spirobranchus giganteus )

You see color spiral tentacles and a cap of the worm's house-tube. The yellow spirals are the worm's respiratory organ. The body of the worm constantly hides in a calcareous tube, that is embedded into coral.

This worm has chosen the Fire coral (Millepora) . After touching this coral the burn remains for a day or two. I'd already touched fire coral so then I wasn't touching anything and was focusing my camera hanging in water, that is not easy.

 

Underwater photography - Spiral Tube Worm

Spirobranchus_giganteus in Wiki