Question

If Teresa's daughter is my daughter's mother, what am I to Teresa?

Answer 1

What is your gender?


let's first look at the case where you are female:

 
my daughter's mother would then be... just you.

so then you are Theresa's daugher!


if you are male then

your daughter's mother is your wife (let's assume for now)

so Theresa's daughter is your wife,
and Theresa is your mother-in-law and you are her son-in-law.

This is all assuming the "traditional family"; in a non-traditional family you would just be the father of her grandchild, nothing else, and probably she would refer to you just with a name..