DTs Phytoplankton comes in a green liquid form. It's not required to feed a feather duster...I'd consider it more of a supplement. Also, if you have other livestock in your tank that would benefit from phytoplankton you can get supplemental feedings done in one foul swoop.
When I dose DTs phyto, I only use about half a capful. The organisms is phyto are "suspended", and frequent shaking of the bottle (kept in the firdge) is recommended. So because it is in a liquid form I turn off my filters/powerheads when feeding. This allows me to see the green liquid suspended in the water column. I'm not sure if this is entirely necessary, but it allows for more of a "spot feeding". I usually only leave the equipment off for about 10 minutes max.
One of my first additions to the tank was a Hawaiian feather duster. It was doing great without phytoplankton, but now that I have other things in the tank I dose to supplement all the other stuff.