Dogs get lumps and bumps in various places, and it is not always clear what the cause of the lump is. It is best to get a veterinarian to examine your dog and decide whether the bump needs attention. A white bump on the dog's ear flap is most likely to be an insect bite, a wart or skin tag, a cyst, or an aural hematoma.
An insect bite or sting could cause a white bump on the dog's ear flap. The site of the sting swells and fills with fluid, which is usually clear plasma that would result in a "white" appearance. Use antihistamine lotion to dry up the fluid and watch your dog's reactions closely for 24 hours in case the bite or sting was from a poisonous spider or insect.
A skin cyst is a collection of dead skin cells held together by sweat or plasma. Both types of cysts may look like white bumps, because of the absence of blood in the cyst, and these are not harmful unless they develop infection. Cysts usually either disappear eventually, or rupture and then heal. Ignore the cyst unless it appears to be causing the dog pain, and if it ruptures, clean the area with a mild antiseptic and apply a healing ointment.
Sebaceous cysts are caused by the collection of sebum, an oily substance produced by the dog's sebaceous glands to moisturize its skin and coat. The sebum passes from the glands to the hair follicles, and occasionally the follicle is blocked by dust or dead skin and a cyst forms. These cysts look like pimples with white heads, and will produce a discharge if squeezed. A sebaceous cyst will heal itself over time, but you may need to consult with the veterinarian if your dog appears to be in pain, the cyst grows rapidly in size, or it ruptures and shows signs of infection.
An aural hematoma is a lump on the inside of the dog's ear that is the result of the dog shaking its head and rupturing a blood vessel. The section of ear swells and fills with fluid, and the hematoma can rupture and develop infection. Although hematomas do eventually dry up and disappear, the dog may be uncomfortable while it has the swelling, and it could end up with a "cauliflower" ear from the scarring that may occur once the fluid dries up and the swelling goes away. The dog may shake its head because of an ear infection, and the veterinarian can address the cause and the hematoma at the same time.