You are hung up on FST. It isn't necessarily a FST. But, it would be illegal motion, if a lineman is moving at the snap, or a back is moving forward at the snap, or more than one back is moving at the snap.
I read your post as this was the end (wide receiver), moving, as in adjusting his weight significantly enough to be able to get a quicker 'jump out of his starting block' just before, and as, the ball was snapped. If that isn't the case, then we're not talking about the same thing. If you are talking about an end (or an interior lineman) shifting his weight to re-balance himself and get re-set, adjusting a foot significantly and getting re-set, pointing or otherwise signaling with an arm(s), rotating or lifting his head BEFORE the snap, and he is motionless at the snap, then you have nothing. A 'one-off' weight shift that isn't complete at the snap, or a 'point' in which the pointer doesn't get his arm tucked back on his thigh, an end (WR) that is turning his head - those kinds of things, are nothing, either (although I would have a 'talk to' with them to get them to get all of that done and be still at the snap).