Diagnosis
The lens has a soft plastic coating that's very easy to mar with rough cloths or cleaners. Those smudges are why the night vision might look hazy or the daytime video seems less sharp.
How to Fix It
Power the camera off and unplug it first. You'll need a microfiber lens cloth, the kind used for eyeglasses or camera lenses. Gently brush away any loose dust with a clean, soft makeup brush or a blower. Then, breathe lightly on the lens to create a tiny bit of moisture. Use the clean, dry microfiber cloth in a gentle circular motion from the center outward. No liquids needed for most smudges.
If That Didn't Work
For stubborn grease, dampen a corner of the microfiber cloth with a tiny drop of distilled water (not tap water, which has minerals). Wring it out so it's barely damp. Wipe the lens gently, then immediately dry it with the dry part of the cloth. Do not spray anything directly onto the camera.
Heads Up
Never use window cleaner, alcohol, ammonia-based products, or paper towels. They will permanently damage the anti-glare coating on the lens, making all your video look foggy. Also, avoid touching the infrared LEDs around the lens with anything wet.