The brown gunk and the radiator light are two issues.
I would flush the fluid and recheck later to see the condition of the coolant.
The "dim" radiator indicator is a sure sign of a failing sensor. Corrosion eventually does this in.
You can clean the contacts for a temporary cure, but it will just come back weeks or days later.
Replace with a new one. Easy 5-minute deal. Good time to do it is during a flush and refill...or in my case, when the heater hose burst and I lost all coolant anyway!
