I assume it’s in an air locked carboy and aging? If so, I just dealt with that problem after advise from folks here.
It was a yeast colony. I dabbed the growth out with a paper towel. Wiped down the inside of the carboy neck with a k-meta sanitizer soaked Q tip, spritzed k-meta sanitizer in the neck then carefully topped up to within 1/2” of the new bung and airlock. That was probably two weeks ago and so far so good.