Well you might reference NFHS: 7-1-7, which describes a False Start, as;
a. A shift or feigned charge simulates action at the snap.
b. Any act clearly intended to cause B to encroach.
c. Any A player, on his line between the snapper and a player on the end of his line, after having placed a hand(s) on or near the ground, moves his hand(s) or makes any quick movement.
NFHS: 7-2-6 defines the requirement that " ALL 11 players of A shall come to an absolute stop and shall remain stationary simultaneously without movement of hands, feet head or body for at least one second before the snap..
NFHS: 7-2-7 provides an exemption for, "Only 1 A player may be in motion at the snap"... and then provides a list of requirements that player must satisfy for that movement to remain legal.
If YOU judge the player, you've observed to be in motion, to have violated any of the 3 actions described, you likely have a "False Start". If not, and that player continues in motion through the snap to where it becomes "illegal motion", it would seem Illegal Motion would be the proper call