add cc glue gun to the footage layer, and then add cc composite.
set cc composite to composite original 'behind' and uncheck the RGB only option. this will put the footage layer in behind the glue gun effect (or you could just duplicate your footage layer, which ever you prefer)
in glue gun, animate the brush position to move from the eye down the face as needed. also, set stroke width, density and reflection as needed.
in the style settings, you can change the style to 'wobbly' to make the tear less of a smooth line, adjust wobble width and height as needed, and you'll probably want to set the wobble speed to 0.
if you want to adjust the opacity of the tear (and you used cc composite after the glue gun effect), you can add the transform effect between glue gun and composite and reduce the opacity setting.