What is this soft plastic like material that has been added to the window, and how best to remove it?

The previous owner added this soft, plastic like material to the window frame and I'd like to remove it. I can get parts off with a knife, and when I do it is flexible and soft.

I believe this is probably a common material, but I'm struggling to identify it. What is it? And how best would you go about moving it from the window?