Cedric and my fantails have alway got on well and he is big at over 3 inches long, so I dont think he will be eaten...However they are not used to seeing him like this "openly" feeding so they have give him a curious nibble but I dont think he tastes too good as they swim off straight away!! If Cedric felt threatened his shell would clamp tight shut!
I have just set up my nursery hatchery and the fish are more interested in this at the moment!
Never keep clam with fish with teeth!!! and certianly never keep them with clam eaters such as crab, lobster and even some kind of shirmp!