This is not new, in fact, it's probably been identified several times since the original video cropped 12 years ago, up back in 2007:
But it just popped up again on Facebook, labeled as a UFO. I recognized it, and so tried to look it up again. It took me about half an hour to track down the video! So I'm just writing this post to save people time.
The small jet is an X-45A, an unmanned aircraft. It was an early proof-of-concept vehicle, and was smaller than the later versions of the X-45. Only two were built, and both are in museums now. It's a lozenge-shaped plane with dark wings that fade into the sky.
This video from NASA shows a close-up of the formation flight with the F/A-18 at 4:24