From the picture, it could be that Lemon has a papilloma (benign tumor) or another kind of benign tumor. In a senior cat, there is always a chance it could be a malignant tumor. Any suspicious lumps should be examined by the vet. The vet can perform a test called a fine needle aspirate on it. To do this, the vet sticks a small needle into the lump to get a sample of it's cells, and examines the cells on a slide under a microscope. It's usually painless to the animal, and relatively quick to perform. I would have Lemon seen by a vet in the morning to have this lump examined and diagnosed so it can be treated as needed.