White stuff inside coolant tank cover?

[g300d]

I was gearing up to do the tank vent valve when I decided to check coolant and oil.

Opened the expansion tank cap and ...white stuff on cap and in in tank!

No trace on dipstick, none under oil fill cap.

Truck runs fine, I monitor coolant temp all the time via cluster and highest it gets is 93-96 on hot days in traffic.

What's wrong with my X this time??!