My child is 16yrs old,she has a baby and doesn't live with her mother anymore either,What can I do about not paying support to her mother?

Go back to the Court that issued the support order and explain the changed circumstances.

Who does she live with?

Agreed, you have to pay, till the court order is changed, So you will need to file a motion to modify or end the payments.

Also, depending on the state ( if in the US), just because she has a baby, does not mean she is allowed to "move out"

Who does she live with?

She lives with the baby father.