This appears to be a current issue with Moho 12/13.5 when using bitmap/raster masks. As stated by Lukas it should be 'resolvable' if you disable GPU or preview the render, or when you export.
Also, to embed images in this forum you need to use the embed code i.e.
In addition to the above info, if you need a razor sharp mask (or even one with a textured edge,) I recommend using a vector layer for the mask. You'll have more flexibility for animating it, and it should render faster too. A lot of the 'bitmap-based' Moho animations I've done at my workplace and in personal projects actually have texture-filled vector shapes and vector masks wherever I can use them because it's more efficient.