Any idea what this could be?
Welcome to Galaxy Zoo, Katavitje!
That's a #satellitetrail, a kind of #artifact. When a satellite (or other fast moving object, an airplane, for example, or a meteor) passes through the field of view of the camera, it will leave a 'streak' like this. Moving quickly, it will generally appear in only one color. Green satellite trails are fairly common in GZ, but you can also come across red ones, and blue ones (but you will never see a magenta one, or a yellow one; can you work out why not?)
Happy hunting! 😃