Look at it this way. If the car is old enough to need a new alternator, then you are probably overdue for new coolant anyway. Coolant wears out the same as every other fluid in your car. It’s the longest lasting, but it still needs to be replaced eventually. But if I were you, I would’ve bought an aftermarket 100% aluminum radiator to prevent this in the future.

If your tires are all on and secured, drive forward.

You could sand it off. The brake pads will polish it back to smooth. Or you can see if the brake pads will take it off.