Weird object appears in every project

Hello, 

I hope someone can help me. I really don't know how to look for a solution for this problem as I don't know what the heck happens. Recently I worked on a video and made a mask from a bird flying across the sky. Since then I keep getting a piece of this bird in every composition I make and even in new projects. Its kinda just stuck to After Effects and only visible when there is no other layer visible at the moment. So it's not a super huge deal as I can easliy 'cover it up' but I still would really like to know what that is and how I can get rid of it. Hope that someone knows ehats up, thanks already in advance! 🙂