[chiefofgxbxl]

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.

[0] https://youtu.be/TXMGu2d8c8g?t=2729

[lutorm]

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.

[x2f10]

Whoa - thank you for pointing out the solid oxygen. That is mind-blowing to me! Super cool!