Odd foam

First time posting. Have been syruping now for 6 years. Self taught other than incredible information gleaned from Maple Trader. This year 21 taps, 130 gallons to 4 gallons syrup on sheet metal parlor stove converted to boiler. Since starting I have 3 other people now boiling sap and enjoying real syrup. Anyway, the one friend sent me this picture of his Norway maple in his yard that he taps. After he pulled his taps about 10 days ago, the very next day on this one tree this mysterious foam appeared below the tap hole. I have not ever seen that, any ideas? Thanks for your information, in all areas.