The common clinical causes of a green hard lump on the arm include sebaceous cyst and dermatofibroma. 1. Sebaceous cysts: this disease refers to a kind of sebum gathered under the skin due to the blockage of the opening of the sebaceous gland of the hair follicle and the formation of small cysts with clear boundaries, round, smooth surface, and some of them may be manifested as a hard greenish-colored lump on the arm. If the size and appearance of the cysts do not change, usually no special treatment is needed, and observation is sufficient. 2. Dermatofibroma: It may be a reactive proliferation of fibroblasts triggered by tiny skin injuries, rather than a true tumor. Mostly seen in the extensor side of the limbs, the clinical manifestation is a slow-growing round or ovoid solid nodule, the surface is smooth or rough, usually no special treatment, observation can be. There are many other causes of hard greenish-colored bumps on the arms, if you have this kind of situation, it is recommended to go to the hospital, the doctor will take corresponding measures according to different situations.