DIAGNOSIS That's either external dirt/condensation or, more likely, internal fogging from a compromised seal.
FIX For external grime, power off, wipe gently with a microfiber cloth and a tiny bit of lens cleaner. For internal fogging, you need to dry it out. Remove the camera, bring it inside, unscrew the lens ring, and let the unit sit in a dry, warm place (like near a router vent) for 24 hours. Reassemble tightly.
IF THAT DIDN'T WORK The seal is permanently damaged. You can try a thin bead of clear silicone around the lens housing seam before reassembly for a DIY fix.
HEADS UP Never use harsh chemicals or excessive water. The lens is plastic and scratches easily. The IP65 rating is void if the housing has been opened.