Sterling Silver and Copper tarnish with exposure to oxygen and moisture, please store in an airtight bag. Over time your copper piece will darken, this is natural and is to be expected.
Copper and even Sterling Silver react differently to each body's chemistry. If a green mark is left on your skin, it is completely natural and harmless, and is a sign that you have an acidic pH. Simply wash with soap and water.
You can seal your metal piece with clear nail polish to avoid green staining and oxidation.