That’s what the plug always looked like in the #5 cylinder of my 71 Maverick. It had issues.
Had a 99-Malibu.
Plugs and coils kept corroding and needing replacement.
Couldn’t figure it out.
After I sold it, I recalled that I had removed the failing under-cover (big plastic sheet covering the bottom of the engine).
Every car wash would shoot straight into the air intake.
The wife would always go through those conveyor belt car-washes where you left the engine running.
Lesson lernt.