Back in 1991 I bought a used Saturn that had a similar rust colored powder under the oil fill cap. It turned out to be STOP LEAK. It didn't matter to me, I had the engine rebuilt and the mechanic found alot of questionable additives in the other fluids as well. And I bought that car from a well known dealer in Dallas, TX.

 

I also found rust in filler tube/cap. Removed valve cover which inside was covered in rust. Now wondering if head gasket is compromised. Engine has developed an unusual vibration over time so maybe that also is an indication of bad head gasket. Still wondering about all of this.

 

You probably have a blown head gasket causing water in the oil pan. Muddy looking oil, growing oil, coolant disappearing, insulators of spark plugs the color of coolant, puddles of water under the tail pipe. The "unusual vibration" is most likely a death rattle from lack of attention to the gasket problem.

 

On many engines, especially ones that have a "dry sump" such as the Porsche Turbo we had years ago will get a light brown to milky build up inside the oil filler tube and inside the cap. This is due to condensation when the oil gets hot and drives the moisture out but cool air going by the area where the oil filler tube and cap are located will make the moisture condense along with some oil vapor.

The stock HHR has that long neck that sticks up through the air cleaner so it is a great candidate for condensation. If you are concerned, I would suggest draining the oil after getting the engine up to temp, into a large flat container to see what comes out. I would suggest taking a gun cleaning rod with a clean patch and run it inside the drain plug hole once all the oil stops. On mine it took about 2 hours and when I looked at the patch it was pretty clean so no sludge or goop in the bottom of the pan.