Sludge in coolant

Mistake it's 2.7l not 2.6l.I bought this car not to long ago everything was going fine until we got our first cold night here in MN. the next morning the heat would not get warm until I started going hwy speeds or revving up my engine. I checked the oil that was good and clean too. next I tried to check the coolant but couldn't get the radiator cap off, as with this make of car it's connected to the reservoir so I could tell from looking on the outside and tapping that the coolant was low. went and picked up the right coolant Zerex g-05. When I went to open the radiator cap there was a yellowish orangish sludge on the cap and in the reservoir. I am curious if there is an issue, to me this isn't normal. I have attached pictures of the radiator cap, the reservoir before adding coolant, the reservoir after adding coolant. Any help, questions, ideas are greatly appreciated!