I would venture to say you don't want to see any cracks in your filter housing. I suspect it is just a superficial crack at this point, and not all the way through, otherwise you would be experiencing a water leak correct? Is the filter old, or still under warranty? If it's an old filter, and the crack is just a minor stress-crack on the outside, perhaps you can get by with some epoxy (JB Weld) or something to help keep it from getting worse.