Illegal Block?

[Reffing Rev.]

The fact the "flanker", who being in motion it is assumed is a back, blocks the DE is not in and of itself illegal, providing the contact is legal.

Any back, a player not on the line of scrimmage is prohibited from clipping, blocking below the waist, or blocking in the back, regardless of the free blocking zone.

He certainly is suspect for committing a foul, but it could also be completely legal.