The gunk in the airlock is nothing to worry about. If I were you, I would let it ferment out as is. You just did not have enough head space in your fermentor to handle the fermentation. At least you had the sense to put a blow off on it, unlike me who simply cleaned out the airlock and reinserted, only to have my fermentor blow its top an hour later. That batch turned out fine.
Drinking - red ale, Honey Wheat, Chamomile Ale, Coriander Mead., Chocolate Porter & aging 6-pack of Smoked Porter
Kegged - Hefeweizen & 80 minute IPA