Only the finest particles, phytoplankton and fine, very small, bacterial aggregates, make it to the bottom of the food groove. These particles are conveyed directly to the mouth where they are eaten. It is important to realize that the worm eats food based only on two things. First, the particles must have the correct density so that they will flow in the water currents in such a way as to impact the surface and stick there. Second, the particles must be the correct size to allow them sink to the bottom of the food groove. If they are too small, they likely will not have enough mass to even impact the mucus with enough force to stick. If they are too large, they are either simply rejected, or used to reinforce the tube. Only if their sizes are, as Goldilocks, said, “Just right,†will they be eaten.
The animal makes not overt choices in what it feeds upon. It neither tastes nor sees its food. However, it uses a sophisticated piece of organic machinery to catch and sort small particulate materials that are about the same size as an average piece of phytoplankton. In fact, the crown of a feather duster worm is well designed to be a phytoplankton trap. To thrive, these animals need plenty of phytoplankton in their water to feed upon, for that is all they normally feed on, and indeed, it is all they normally have as food.
Hmmm.. so I'd suggest turning the pumps off for half an hour and using a syringe or Turkey Baster to gently squirt something like ReefRoids (expensive, but a lifetime supply for your worm) into the duster.
You might also leave the pumps on but skimmer off and clean you glass with a magnetic cleaner which will introduce lots of fine bacteria etc into the water for it to catch. I get all sorts of critters coming out to play when I do that