What would your mom's cousin's husband be to you?

he would be your cousin in law *once removed*.

"Once removed" is a term for cousins who are separated by a generation, such as your mom's cousin is to you(and also her husband to you as well).

Technically, you all are "first-cousins"(even with the removals), but you and your mom's cousin's children would be second-cousins, just fyi :)