Often in RE:Flex you can set Boundary shapes ( a from and to shape – where pixels outside shape won’t move)
I only see one frame but you probably need to make this somehow in two or maybe 3 layers.
Seeing only one frame it looks like to try to make a boundary shape that protects the pen until the edge of the black gradient as one of the layer might work (so a closed shape that outline the pen). Then you make another morph that use as boundary a shape that goes around the finger and the thumb and catches the pen
https://www.revisioneffects.com/bugreports/layered_morph.jpg
pen boundary shape in green
If that does not work, sometimes you need to make clean cut outs to morph, and sometimes if the top deformation leaves some small holes when you recomposite all, RE:Fill is your friend.
Pierre