It could be a film yeast. Film yeasts won't cause too many problems - at least not quickly. I've only dealt with them in cider and there I added sulfites and racked the cider from under the yeast. Film yeasts also need oxygen to grow, so it would be a sign that there is oxygen in the headspace of the barrel. If it is aging in the barrel, it really should be full right up to the airlock.
Of course, it might be something else?? Does it kind of break apart into jagged sheets when disturbed?
No matter what, if it tastes good don't worry about drinking it. No human pathogens can survive in beer due to the alcohol.