Where did the mold form? As long as it's not underneath the little bobber thing, you should be fine since it keeps the air out. If it's in the stem, you may have gotten mold spores into your beer. Either way...mold shouldn't really effect your beer. Ive had fruit beers get mold on the fruit in secondary and I just rack the beer from underneath it. No problems.
If you're using water in your airlock, that may be the issue. I just have a bottle of cheap vodka on hand and fill my airlock with that.