No. Clark's nevus is a type of atypical (dysplastic) nevus or mole. Many dermatologists feel these are benign, some experts think they're precursors to melanoma. Having many clark's nevi always increases the risk of melanoma. Becker's nevus is a benign (normal) nevus that occurs on the trunk, often in boys and men. They're often large and light brown. Sometimes hairs can grow from it (after puberty).
No. If you have several clark dysplastic nevi, you are at extra risk for malignant melanoma because of your genes, but you can still probably avoid major problems just by keeping a close eye on your skin like everyone should. Becker's nevus has almost no premalignant potention; 1 man in about 200 has one and several hollywood men clearly like to show theirs off.