Because this is a malignant tumor and not normal tissue, it should not be dismissed and treated as you would treat a normal tissue wound. As for equine melanoma being "generally harmless", that is not an accurate statement. While human melanoma differs from equine melanoma, it is still a malignant cancer in horses that can spread systemically and can behave in varied ways. While some melanomas may behave like benign growths, a number equine melanomas spontaneously develop into very large, multinodular, advanced and infiltrative tumors that metastasize to other locations. All equine melanomas are malignant cancers and even the smallest most seemingly benign tumors can at any time develop new patterns of growth.
When an injury to the tumor such as this occurs, a call to the vet is warranted. No two cases of melanoma are identical, and your individual horse will benefit most from medical evaluation of the injured lesion by a vet with expertise in treating melanoma. At the very least, a call to the vet is warranted.