Learn English – Non-related cousin

There isn't any family relationship between you and Mark, except

• the cousin of your cousin is yourself or your sibling
• the cousin of your cousin is also your cousin if his mother is the sister of your father
• the cousin of your cousin is your second cousin if his parents and your parents are cousins (third cousin if the parents are second cousins, and so on)

A term what many people use is grand-cousin. But that's wrong, though it is used ambiguous in colloquial, at least in my mother-tongue. But think of that:

• Your grandparents are the parents of your parent
• Your grandaunt/granduncle is the the aunt/uncle of your parent

This leads to

• Your grand-cousin is the cousin of your parent.

If no blood relation exists there are only few ways how a non-related person can be your cousin: E.g. the person is your step-cousin. But this doesn't work here, because:

Step-cousins are either stepchildren of an individual's aunt or uncle, children of one's step-aunt or uncle, or half-siblings of one's cousin. ^(WIKIPEDIA)

On the WP page you find a lot of family combinations and what they are called (e.g. Cousin-in-law: spouse of an individual's cousin), but I didn't find the relationship you mention. (Correct me, if I overlooked it)

Conclusion: I would call him cousin of cousin.

Considering that the proper term for your spouse's brother or sister would be your co-brother-in-law or co-sister-in-law and though this term would be highly uncommon, it could be suggested that their kids would be your co-niece-in-laws and co-nephew-in-laws. However, the term co-sibling-in-law is believed to have stemmed from translations of South Indian languages, which could be the reason for its wordiness.