Could be residue from something that was on the bulb that burned off and coated the inside of the lens. If it's not moisture, try to get some thing soft and clean the inside of the lens housing. A microfiber cloth attached to a rod or similar to wipe down the interior. Then blow out the interior with a compressed air. If that does not do the trick then the lens itself could be bad and it would have to be replaced. Never have seen a lens go bad before so that is unlikely though.