5e combat is rigid, however there is a suggestion in Chapter 9 Combat to allow players
"IMPROVISING AN ACTION"
Quote:
When you describe an action not detailed elsewhere in the rules, the DM tells you whether that action is possible and what kind of roll you need to make, if any, to determine success or failure.
So here is what your DM tells you about attempt to "knock off a mask" from a creature who is actively fighting:
1. It is impossible to both to inflict damage to such creature AND do the attempt
2. To success on "knock off a mask" you would need to make a successful DC12 Dexterity(Sleight of Hand) check.
Note: an improvised action is an action, so you can't do another action on the same turn, but can still move and do bonus action.
Picking up an object not worn or carried (i.e. knocked off) classify as one free action per turn.
