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It seems to be nevus sebaceous. It is a benign cutaneous hamartoma. You don't need to worry about it.
It looks like syringocystadenoma papilliferum. It is a rare benign adnexal tumor. Even though it is called as a childhood tumour as it usually appears at birth, during infancy or around the time of puberty, it rarely appears in adults. Basal Cell Carcinoma (BCC) development has been reported in upto 10% of the cases. In the majority of the cases, there is a coexistent nevus sebaceous. Squamous cell carcinoma (SCC) may also develop, but much less frequently. Till now, only two cases of verrucous carcinoma in conjunction with Syringocystadenoma papilliferum, have been published. Ductal carcinomas which arise from Syringocystadenoma papilliferum have been reported as well. An ulceration or a rapid enlargement is indicative of a malignant transformation. Syringocystadenocarcinoma papilliferum is a malignant counterpart of syringocystadenoma papilliferum. The diagnosis is clinically suspected and histologically confirmed. Due to the risk of a malignant change, a prophylactic surgical excision, followed by a detailed histological examination, is the treatment of choice. Considering the size and the ulceration, an excision was done with a 1cm normal margin, but it proved to be histologically benign and a malignant transformation of the tumour was not seen, even after a follow up for a period of ten months. You should see a dermatologist to have a prophylactic surgical excision.
It is not melanoma but syringocystadenoma papilliferum. It is more likely than nevus sebaceous according to your photo. Syringocystadenoma papilliferum is benign, but it could be transformed to malignant one. You should have a surgery to remove it.
I don't see any malignant transformations now. Also the cancer grows very very slowly even though you have it. 3 months should be alright.