If the ball is still in the scrum then it is illegal, unless somehow the flanker has become the player in the scrum whose hindmost feet are the furthest back (see rule 20.10 (c)), obviously this is normlly the Number 8.

Once the ball leaves the scrum (possibly through his channel) then it is fair game.

The question you have to ask yourself is:

Did the ball come out and then the flanker unbind and pick up? If yes, play on. Or, did the flanker unbind before the ball was out and pick up the ball at his feet? If yes, it's a penalty.

Thanks,

Antony