Anyone have any idea what would cause this?

Just realized this is in the wrong place. Sorry please move or delete.

Corrosion from sitting.

I drove it the day before. So it has really been sitting.

Look at the rust on your valve guides, its all forms of corrosion, whether from sitting or just moisture from sitting overnight, its corrosion from water in that area from one reason or another.

Usually white specs or buildup in the exhaust is from coolant being burned and crystalized, then clinging to carbon deposits in the exhaust path and building up. Could be a small amount or just regular moisture if the car isn't driven super often, and it's whatever minerals are in the water vapor crystalizing in the exhaust path. Either way, it's moisture related.