I know ABC Reef had some that fit your description.I believe they are filter feeders though.(i.e. water column)I have one that is mostly pink with a touch of black,that doesn't have the short tentacles on it so I assumed it would be a sandsifter.However,doesnt even go near the sand.It seems to work its way in to rock crevices to feed on settled detritus.No change is size (shrinkage) so I assume it is geting at something.
I think for sandsifting cumcumber worms ,are the black or tigertail type.IME