In the video of the CRS-19 mission of the SpaceX Falcon9 rocket, flat-Earthers noticed something is moving near the exhaust of the second stage rocket. Creatively, they interpreted it as a “mouse, ” and then use it to discredit spaceflight and the people involved in it.
In reality, it was merely crystallized waste from the rocket engine. To know this, we only need to see the part of the video after the one cherry-picked by flat-Earthers. The “mouse” slowly disappeared by evaporation.