Q.
A greenish-blue substance has appeared on the copper product.
A.
That is copper rust, known as verdigris. It is harmless, so please use it with confidence.
It is a compound that is not easily soluble in water, so it is not easily absorbed by the human body and will be completely excreted even if ingested.
To remove verdigris, mix an equal amount of vinegar and salt, apply the solution to a cloth, and gently scrub.