Active galactic nuclei? Quasar?

probably just a star

If you mean the blue-white thing: that’s a star, with a Gaia-reported proper motion (ie, it’s moving across the sky).

If you mean the green squiggly thing: that’s a cosmic ray – a charged particle that hit our detector. We try to remove them from the images, but when they’re on top of a bright background, they’re harder to detect.