If it's normal weight (as I suspect) then it's either an improperly annealed planchet or post-strike discoloration. Given the complete absence of mint luster, I lean strongly toward the latter.
If I'm not mistaken a missing clad error will weight less due to the missing clad. Have you weighed it? Just looking I'd it's not missing but the copper wash thing sounds correct to me as well. In fact to me it looks stained.