My 4 year old son has 2 pet snails that we obtained from our yard. 1 of the snails has been getting a white film/coating covering its opening. I have gently used a moist cotton swab to rub it off, and then the snail comes out of its shell and it's "business as usual". What is this white film? Am I doing the right thing by removing it? Is there something we can do to prevent it from re-occuring?