I know they eat worms ... not sure about Featherduster worms. Sadly my mandarin died last week, just after my first batch of clown fry hatched. Still not sure why he died, he was not thin.

They primarily just eat copepods. Even with a 10 gallon
refugium, the copepod population raises and falls over a month. As for amphipods, my tank is crawling with these. The wife calls them saltwater cockroaches LOL. I never saw my mandarin eat amphipods, just copepods. I feed him copepods (that I buy at the
LFS), live
brine shrimp, live baby brine shrimp (which I hatch every other day) and sometimes flying fish eggs.
If you want to buy one. Have the LFS worker feed him live brine shrimp in front of you. If he eats ... He will most likely eat anything floating by him !
I came close to buying another mandarin this week ... but changed my mind. I think I will take a break from them until I get better at raising baby clowns.