Propane is stored in liquid form and then vaporises inside the canister. To do this it requires heat, which it absorbs from the surroundings. Normally this is scarcely noticeable. However, if you are using a lot of gas or the canister is nearly empty, the temperature of the contents may fall below 0°C. The moisture in the air will then condense as ice on the canister. This in itself is not dangerous. In the worst case, the gas pressure might drop so low that the flame is extinguished.