some others can prob answer better but they seem to like very fine particulate food/plankton. Jake Adam’s feeds his x-mas worms a plethora of prepared fine foods (he has a reefbuilders video on his coral food).
frequency of feed is hard...many reports that they look ok but are actually starving or don’t make it past a year. I would think size and maturity as well as fish population of the tank all matter in terms of frequency. I also would think ever present food in the water column is what they want. Some feed phyto daily and others x times weekly...maybe start where you are comfortable and monitor water quality as well as worm behavior (is it always out and feeding and defecating?). I feed my tank a capful of concentrated phyto daily to keep dusters fed (at least I think) as well as roids and other particulate foods when I feed corals/fish.
just curious is it a Hawaiian duster or another kind?