It looks like you've tracked it to the eye.
With Mocha AE, you could track it to her cheek instead.
Once you have a track for the cheek, you could also create some spline shapes that could act as a mask for the ink - so you can remove the parts of the ink that you don't want (rather than extending them)
A simple mask will hide the edges so there's no need to fill anything. Your tracking is only fair. komaruyama said you need to track a bigger target. You'll also probably want to work on the color grading a bit more because right now it just looks like you stuck a black blob on the video and it should look like the ink is actually on the face. Start with opacity adjustments.
To make this look more realistic, I also would warp the layer to follow the curved surface of the face.
This tutorial shows how to do this in a stabilized precomp with mocha and MochaImport+
The tutorial is a bit older and says that you also need Red Giant Warp for the stabilized precomp workflow. But The the current version of MochaImport+ can also create stabilized precomps with other corner pins.
Here is another tutorial showing face manipulations with this workflow:
And finally here is a tutorial showing how to create the stabilized precomps with the current version (since the user interface changed a bit):