This is more about animation than graphic display. I believe the hockey mask is part of the chainsaw graphic, which is more 3bpp graphics data. To that end, you could probably find it in FF6Tools or YYCHR and simply erase it, since you're not seeking to move it to another animation.
For information on how to navigate 3bpp graphics in YYCHR, you should check out Mutteo's harp tutorial thread. It's very helpful to find the data and see how to work with it.
https://www.ff6hacking.com/forums/thread-4013.html
In one of the images in the middle, YYCHR is open to address 1343B0 and you can see the autocrossbow at the bottom of the shot. I bet the two chainsaws are near there.
EDIT: If you go to 135C70 in YYCHR and set the display mode to 3bpp, you'll see the mask in the upper left of the window. The top of the mask is in the second tile row, it's the first two tiles there. The bottom of the mask is in the first tile row on tiles 2 and 3, it's connected to the top of the chainsaw handle so you'll need to be conscientious as you're erasing it.
For information on how to navigate 3bpp graphics in YYCHR, you should check out Mutteo's harp tutorial thread. It's very helpful to find the data and see how to work with it.
https://www.ff6hacking.com/forums/thread-4013.html
In one of the images in the middle, YYCHR is open to address 1343B0 and you can see the autocrossbow at the bottom of the shot. I bet the two chainsaws are near there.
EDIT: If you go to 135C70 in YYCHR and set the display mode to 3bpp, you'll see the mask in the upper left of the window. The top of the mask is in the second tile row, it's the first two tiles there. The bottom of the mask is in the first tile row on tiles 2 and 3, it's connected to the top of the chainsaw handle so you'll need to be conscientious as you're erasing it.