Looks like the space shuttle.
Isn’t the F-18 a swingwing design? If so, its flying fairly slow since the wings are in the forward position. I’m no aviation nut, so somebody chime in.
In the absense of any other info my guess is that the F-18 is tracking a drone or something like it—given its size.
Don’t get me wrong, I’d love it if some fine fellows from outer space landed and said hello. But I don’t think this is it.