"Lens Dirt" is a graphics effect used in Experimental Rust. It is an effect designed to make Rust look more realistic from the first-person standpoint by making it as if the imaginary camera that is the player's eyes had dirt on them. It makes the imaginary camera look a bit fogged up and dusty, and as a side effect can make everything look bright, even at night. Fortunately, this can be turned off in the Settings in the Experimental Rust (F2 key by default.) The F2 Settings panel will only work when in a world. It will not work in the main menu.
Note: Lens Dirt is ON by default.