As mentioned in a previous post, this happens when there's not enough headspace in the fermenter (too full) or when fermentation is allowed to happen uncontrollably. In the latter case, is more likely with with higher gravity beers. Do you use any sort of temperature control? For an inexpensive alternative look up "swamp cooler". If this were me, I'd clean up the goo on top with Star San soaked paper towels and replace the airlock with a freshly sanitized one. Happy brewing!