I support Reinz's explanation. Unless the Manufacturer is intentionally applying some type of compound, the white substance sounds like aluminum corrosion byproduct. The mechanism is galvanic corrosion, and yes, the electrolyte can be formed from moisture condensed out of the air. In this context, the aluminum frame material is anodic to the steel/stainless-steel screw material. The thing about corrosion byproduct is that it is less dense than the metal and hence occupies a larger volume. This explains the seizing and lock-up of the screw. The Manufacturer should make some effort to coat the thread and/or prevent air from getting to the threads interface: lithium grease comes to mind, applied to the threads with a toothpik. I would predict that grip screws that are not removed over a period of many years (owners just leave the OEM grips undisturbed) will likely become frozen and unremovable, and how quickly this happens should depend on the environment: faster in Florida than Pheonix.