In the future you should consider creating a motion path for a Particular emitter instead of a mask. Masks are limited to 2 dimensions (X,Y) while a motion path on a 3D layer can move in 3 dimensions.
Consider creating a 3D Null layer and animating it in 3 dimensions. You can use the same pen tool to adjust the motion path once you’ve created it, the only difference being that you can adjust it in 3 dimensions. You then need to link the position of the emitter to the position of the Null with expressions.
An alternative is using a light as an emitter for particular, which requires no expressions to link the position.
Darby Edelen
Lead Designer
Left Coast Digital
Santa Cruz, CA