Actually, I think the rule is defined pretty well. Here is the definition from the NLF rule book:
GRASPING FACEMASK
Article 5
No player shall twist, turn, or pull the facemask of an opponent in any direction.
Penalty: For twisting, turning, or pulling the mask: Loss of 15 yards. A personal foul. The player may
be disqualified if the action is judged by the official (s) to be of a flagrant nature.
A.R. 12.12
Third-and-10 on A30. Runner A1 runs to the A33, where he is tackled by B1, who incidentally grasps A1’s
facemask on the tackle, but it is not a twist, turn, or pull.
Ruling:
A’s ball, fourth-and-seven, on A33. No Foul
In real time, I think the refs made the right call as it definitely looked like a facemask. But reviewing it in slow motion I don't think it was, according to the rule as stated above (and confirmed by the example given which says as long as there is no twist, turn or pull then it is not a penalty).
While I'm glad the penalty was called, this is the exact type of play that supports my belief that any play should be able to be reviewed.