I´ve never seen my softies feeding... and I've fed with brine shrimp (nauplii), brachionus, phytoplancton and some comercial liquids.
Some corals like Xenia have so atrophied digestive system that they probably couldn´t eat anything.
For some like stereo, sclero, or dendronephthya... really not known but I think bacteria as food could be very interesting.