Any idea what that light behind Starship was?

The footage alone probably cannot establish what it was. A shape that seems to be behind a spacecraft on screen might be nearby hardware or debris, but reflections, camera effects, and the viewing angle can also make things look like separate objects. Without more views or flight details, those are possibilities, not an identification.

I’d hold off on calling it anything unusual. A livestream image can make a faint feature look solid, especially when there is little background detail for judging distance or shape. A clear sequence of frames, camera context, and other available views would help narrow it down, but the clip as described does not settle it.

Could be something physical, could be an imaging artifact. The useful next step is checking whether the same shape appears consistently across nearby frames and other camera views. If it only shows up in one view, that would change how much weight I’d give the object interpretation.