Id say flush it.

Last summer on a really hot day I got out of my truck after a 20 min drive on the highway and coolant was pouring out from the reservoir.

Turned out the Radiator cap had a bad seal and after I replaced it and topped off the coolant its been good ever since.

I did notice a little bit of the clayish colored stuff both in the reservoir and on the inside of the cap like you did, but i didnt have nearly that much..

I never thought much of it though since the coolant had just been flushed a few years before when there was a headgasket problem.

Maybe I need to flush mine at somepoint.