Interesting find! Earlier in the feed you can see a solid piece of oxygen [0] bouncing off the engine. The particle you mentioned is going past the spacecraft (forward), so I can't see it being solid oxygen. My guess is a piece of dust that is being illuminated, and the camera is picking up the reflection but is blurry, making the particle look bigger than it actually is.
It happens right as the spacecraft is recoiling from the separation impulse, so I'd guess it's something coming off from the top of the stage and passing by the camera.
[0] https://youtu.be/TXMGu2d8c8g?t=2729