Common occurance, called carburetor icing. Air passing through the carburetor ventuti increases in velocity and decreases pressure causing the air to be cooled below freezing temperature. Same principal as air conditioning and refrigeration. Very humid conditions makes thid condition much more severe.
It will not cause damage running this way unless the ice is also forming inside the carburetor thus restricting the air flow and casuing the engine to lose power.
Many times you can shut the engine down and let the carbureor and manifold warm up and often will run normal for quite a while... Gene