Well, wouldn't call it an attack even if nem did eat it.
All nems CAN eat a fish, but most host nems are not a prob unless the fish is weak, and drifts into it.
I have had a few baby naso's swim straight on in to meet their demise, but they were tiny, nem was huge, and obviously clueless to what nems are.
I have seen weak or dead fish pulled into nems by good hosting clowns.