What could be a white lump on my penis?

Several things, But the most likely cause is a sebaceous cyst. These are benign skin lumps filled with keratin and fluid. They can be stable, or enlarge and get inflamed, and sometimes drain on their own. If you have a painful, inflamed, or enlarging lesion on the penis, see your physician for evaluation. Many other benign skin issues also present as whitish lumps.