That rock in your tank looks fairly new or mostly base rock...so I would imagine this is a fairly new set up? Check your water params. It's stressing over something, and that would be my first guess. If the water quality is not where it should be, it probably won't eat much if anything anyway. Probably wouldn't hurt to bury the tube in the sand leaving about 1/3 of the tube sticking out...mine attached itself to a rock, and the goby helped bury it. Also, how long have you had the feather duster, and how did you acclimate it? One thing people don't always consider is that you need have sufficient acclimation times, and *never* let the FD be completely out of the water at any time - even when transferring from the acclimation container/bag to the tank (causes great stress on the integrity of the tube). Phyto is great for feeding, but once your tank is a little more "seasoned" you won't have to worry so much about feeding it, as they are filter feeders and can get a most of what they need out of the water, and excess food (this is why I usually don't strain the excess small particles out of the food when I feed).
Start by checking the water params - including SG (salt content) and let us know what you find.

Wendy