Based on the look and size of it, I'd guess it's likely some sort of bacterial infection. Columnaris can often present as white patches that lead to ulceration/lesions. It's usually more common for columnaris to appear on the lips (cotton-mouth disease) or the dorsal area (saddleback disease), but really it can appear anywhere. It's difficult to diagnose things like this over the internet though, and only a bacterial culture would give you an accurate answer.