The visible coma and tail of a comet transiting the inner solar system (inside of 5 AU and Jupiter's orbit) is mostly the photodissociation products of outgassing water. How does this compare to full moonlight, enough light to trigger coral spawning, the dawn sense of some trees, and reveal prey to predators?
1 Rayleigh unit is 1e10 photons per square meter.
The night sky is 250R, full moonlight is 1e4 R, a bright aurora is 1e6 R. (from this quora post, primary source needed )
The magnitude of the full moon is -13 and is a reasonable definition of "too bright" for an artificial space object. Comets can be as bright as stars; -2 for C/1995 O1 (Hale-Bopp) near its peak, +2 for Halley near its 1986 peak.