Diagnosis
This is almost certainly condensation or water intrusion inside the lens dome, not on the outside. The seal has been compromised, letting moisture fog up the space between the outer weather shield and the actual camera lens.
How to Fix It
Unfortunately, there's no user-serviceable fix for this. The unit is supposed to be sealed. You can try a temporary 'dry out' method: 1) Carefully detach the camera from its mount and bring it inside. 2) Remove the security screw and take the camera body out of the outer shell/housing. 3) Place the shell (the part with the foggy dome) in a sealed bag with a bunch of silica gel packets (the kind that come in shoeboxes) for 24-48 hours. This *might* draw the moisture out. Reassemble and remount.
If That Didn't Work
If the fogging returns quickly or never clears, the seal is permanently broken. The silica gel trick is just a band-aid. The internal components are now at risk for corrosion. You need to contact SimpliSafe for a warranty replacement or repair, as this is a failure of the weatherproofing.
Heads Up
When you remount it, make sure the rubber gasket on the back of the outer shell is seated perfectly and isn't pinched or twisted. Also, ensure the security screw is snug but don't overtighten it, as that can warp the shell and break the seal.