Hi, Jack. You’ll need to roto or key your mask (I’m guessing this is a “face mask”, a real object, not an AE Mask). Once you’ve done that, use S_Glow and use the Mask layer as the Matte input of S_Glow.
Here’s a step-by-step; this is just one way, experienced AE users probably know some tricks to speed this up.
Put your video into a new comp; name it MaskMatte (it’ll end up as the matte for the hero’s mask). Add a new solid white layer. Make it invisible, but select it. Draw the outline of the object you want to glow with the pen tool; you’ll see it turn white at this point. Keyframe that Mask over the whole shot. Now turn off the source layer so you only see the white-on-black matte. (Feather it a little if you like.)
Make a new comp now with just the video — this is going to be the final comp. Drop the MaskMatte comp into it and make it invisible. Add S_Glow to the video track, and in its Matte dropdown, select your MaskMatte layer. Voila, your glow tracks the object!
Let me know if you have any questions, and anyone else feel free to jump in with other ways to do this!
— Gary Oberbrunner