DIAGNOSIS Fingerprints, dust, and condensation are the usual culprits. The lens is a soft plastic dome that scratches easily if you're not careful.
FIX You'll need a microfiber cloth (like the kind for glasses or screens) and a drop of lens cleaner or distilled water. First, power off the camera. Lightly moisten a corner of the cloth, not the lens itself. Gently wipe the lens dome in a circular motion from the center outward. Immediately use the dry part of the cloth to buff it clear. Give it a minute to fully dry before powering back on.
IF THAT DIDN'T WORK If it still looks hazy from the inside, it might be condensation that's gotten inside the housing, which is trickier. Try placing the powered-off camera in a sealed bag with a few silica gel packets (from shoeboxes) for 24 hours to draw out the moisture.
HEADS UP Never use window cleaner, alcohol, or abrasive paper towels. Don't press hard. The protective film I mentioned earlier can sometimes leave a sticky residue—if that's the case, the slightly damp microfiber cloth with gentle pressure should take care of it.