This sounds like a condition called flowering. It is an infection of sorts. best to sulfate the wine put in a carboy and 1 crushed campden dissolved in water per gallon should do the trick then refit the airlock and keep an eye on it. I am sure others will be around to give more input.
Was this a brand new barrel or did you get it used somewhere? It is an infection though.
Flowers of Wine: Small flecks or blooms of white powder or film may appear on the surface of the wine. If left unchecked, they grow to cover the entire surface and can grow quite thick. They are caused by spoilage yeasts and/or mycoderma bacteria, and if not caught at first appearance will certainly spoil the wine. If caused by yeast, they consume alcohol and give off carbon dioxide gas. They eventually turn the wine into colored water. The wine must be filtered at once to remove the flecks of bloom and then treated with one crushed Campden tablet per gallon of wine. The saved wine will have suffered some loss of alcohol and may need to be fortified with added alcohol (brandy works well) or consumed quickly. If caused by the mycoderma bacteria, treat the same as for a yeast infection. The Campden will probably check it, but the taste may have been ruined. Taste the wine and then decide if you want to keep it. Bacterial infections usually spoil the wine permanently, but early treatment may save it.
The flowering does not appear to be occurring in the barrel. When I take a decanter of wine from the barrel the flowers form after a day or so if I don't finish the wine. I hate to keep throwing out the wine. If I treat the barrel as discussed do you think this may prevent the problem in the decanter?
Have you checked the S02 levels of the wine in you barrel? You have much more 02 exposure with barrel so you really have to test S02 frequently and keep them up or you could ruin your wine and the barrel. Once the barrel gets infected there really is no way to fix it. Its wood and microbes get absorbed in there. Tgis isn a reason that wodden cutting boards arent really used antmore cause of cross contamination! You just cant sanitize a piece of wood.
Even though the flowering doesn't appear to be happening in the barrel, I would say the conditions are such that it could happen rather fast. How many gallon is in the barrel? Is it possible to rack it over to jugs? I am thinking a fast transfer and sulfating can save your wine. The barrel would be very questionable in my opinion. I know not everyone has the stuff to check S02 another indicator would be alcohol content, do you have a Vinometer? To see if the % has dropped any?