Hello, Thanks for posting in HealthCareMagic. It seems black spot under your nail which is growing is a fungal infection, However, melanoma can also present similarly and pseudomonal infection should be ruled out simultaneously. Fungal and Pseudomonal and fungal infections can change nail s color to yellow, green or black and these are common in sweaty feet, diabetes, chronic illness or any immunocompromised condition. Typically melanomas will show pigment not only within the nail, but also around the nail bed. This is called Hutchinson s sign.. You are advised to visit doctor for physical examination to diagnose and manage it properly. Hope it helps. Take Care!