Go with Steve’s idea, but to this I would add a white solid layer. Apply an oval mask and set the mask to subtract. Add a feather to the mask.
Next, set the white layer’s transfer mode to add or screen.
Add the turbulent displacement filter, adjust the properties to taste and animate. This will give you a nice organic mask.
Duplicate the white layer and close visibility of lower of two.
On the top white layer bring the transparency down (maybe around 40-50).
Create a new adjustement layer and add gaussian blur and set to around 15 (adjust to taste). Drop this layer between the two white layers in the timeline.
Set the adjustement layer to use the white layer as a track matte.
For the turbulent displace, you will want to play around with: the amount, the size and the complexity. You will animate the evolution.
Hope this helps
Mark