Currently there isn't masking functionality in Edge Animate, but it is something that is being looked into for a future release.
There is a tutorial that I made that shows how to achieve a masking effect in Edge Animate using simple shapes. However, it is not designed to be used against a transparent background. I hope you find the tutorial useful and can apply the techniques to your situation:
I hope this helps! If you have any more questions let me know! : )
Thank you for your help!
there is another cool way to mask any layer/image is by using the clippping tool (shortkey 'c'), you can mask any image and also you can change the size and postion of the mask in properties panel