Was the player in question a back (i.e., not breaking the plane of the center's butt? If not, he is a lineman, and if a linemen moves, it is a false start, which kills the play.) If he was a back, 7-1-4-b-1 says that 'One back may be in motion, but that back may not be moving toward their opponent’s goal line.'
Basically, under no circumstances can personnel in motion be moving towards the LOS at snap – this is an illegal motion foul – live ball foul (i.e., play is not blown dead).
The rule doesn't address feet specifically, only that they cannot be moving toward the LOS. As you described this, assuming he was a back, I believe it was called correctly. If he was a lineman. it should be FST, which kills the play.