[Mrmerlin]

it might be a good idea to swap out the coolant,
also pay close attention to the radiator edge seal,
if it has plastic tanks then it may also be leaking along the seal.

What your seeing is the heavy ends staying behind as the water part of the coolant mixture evaporates.
It might also be a good idea to swap out the thermostat,
use distilled water found at the supermarket to mix with your new coolant