I have this shelled mollusk on the brown porites Xmas tree rock too - did not see any harm from it, no special feedings.
Just for fun - another, no visible shell, retractile mollusk on beige porites Xmas tree rock, was told, that it's bivalve:
Expended polyps:
IMHE, if the flow is not irritating and no chemicals are dripped it the stream nearby, polyps almost always are extended (you are feeding tank by the smallest food, right?):
up to this size in lower light place:
The greenish new growth kind extends less:
again, more - in shaded area:
My brown one closes polyps and, later, sheds the film, in case of harsh new water, or flow directed on it, or handling sometimes.
Do you know anything about what the beige (photo above) and pure green porites with wine colored worms require?
Have die-off on the first, and browning - on the second, after high Mg for bryopsis treatment and setting new light (same wattage).