Hello, currently two kinds of masking are available with Spine.
Clipping, which is a very expensive runtime operation, and can only use polygons:
Clipping - Spine User Guide
And in case you're using Unity, there options:
Blog: Unity SpriteMask and RectMask2D support
However, depending on what you want to do, a clipping mask may not even be necessary if you think about the problem from a different angle. I can help finding a solution if you want to explain your use case (:
