This happened to me on another batch... recipe and picture here: http://www.homebrewtalk.com/f69/dark...ck-ipa-333549/
Blew the airlock off once - I cleaned it up and thought it was done. The next day it happened again and looked like it did in that picture.
a) If possible, insert a blow-off tube instead of an airlock
b) Let it go open until it slows down, and add the airlock then
Given the pressure of CO2 coming out of the carboy, unless you have the beer in a room where particles or other nasties might blow around... it should be okay.
I definitely wouldn't dump it until it's fully fermented and you transfer. If it tastes sour then, oh well... but it just may be your best beer.
The Black IPA in the above link was my favorite beer out of 10-12 AG batches.